Just to note. I didn’t come up with this on my own but I adapted it from the work of someone else. I’ve asked that person previously if he wants me to credit him and he hasn’t responded.
The theory is that originally the mark of the beast meant “man-made torah” in Hebrew or Aramaic while when it was translated into Greek some translators changed it to mean “man-made Christ” (the two are actually connected)
The first thing is that the number is 616 in the earliest manuscripts: https://en.wikipedia.org/wiki/616_(number)
Gematria (or assigning numbers to letters) can get very full of woo and yield too many results (you can find 616 and 666 in too many people’s names) so I want to figure out the shortest and most simple way to get 616 in Hebrew. If these sources are correct maybe you could do the following:
(yod means “arm” or “hand”) (in the previous source they only mentioned the “arm” meaning)
tav means mark
resh means head
yod means hand
vav means affix/add
(all adds up to 616 http://www.gematrix.org/?word=%D7%AA%D7%A8%D7%99%D7%95 )
Looking at the context of the verses  you get the idea of man-made Torah.
So why did some translators of the Hebrew/Aramaic (or copiers of the Greek) change the number in Greek? Since 616 is in the earliest documents maybe the later translators were worried that as Christianity spread the audience wouldn’t understand the meaning of Hebrew gematria and since the Greek letters also had meanings they changed the number to get a similar meaning in Greek (with Christ being thought of as the living Torah) 
In the Greek 666 is represented as chi (600) xi (60) stigma (6) and the meanings are “Christ detached-from piercing” This is a reference to the gnostic version of Christ  It would take me too much space to describe all the animosity between early Christianity and Gnosticism but basically that there are a lot places in the new testament that criticize the Gnostic rejection of the law or “antinomianism”.
Just to include the mainstream theory in the mix as well the other idea is that 666 means “Neron Ceasar” and that 616 means “Nero Ceasar” 
It is important in this connection to note the Hebraisms of the whole of this part of the book, which prove that the writer or—if he himself originally wrote Hebrew or Aramaic—the translator could neither write nor speak Greek correctly. As to the relation of this to the apocalypse which follows see below.
Compare the following Paul quotes Deuteronomy 30 and uses “Christ” in place of “Torah” (one idea you can get is that Christ is the living torah in that he fulfilled the law: Matthew 5:17)
5 For Moses writes about the righteousness which is of the law, “The man who does those things shall live by them.” 6 But the righteousness of faith speaks in this way, “Do not say in your heart, ‘Who will ascend into heaven?’” (that is, to bring Christ down from above) 7 or, “‘Who will descend into the abyss?’” (that is, to bring Christ up from the dead). 8 But what does it say? “The word is near you, in your mouth and in your heart” (that is, the word of faith which we preach):
11 “For this commandment which I command you today is not too mysterious for you, nor is it far off. 12 It is not in heaven, that you should say, ‘Who will ascend into heaven for us and bring it to us, that we may hear it and do it?’ 13 Nor is it beyond the sea, that you should say, ‘Who will go over the sea for us and bring it to us, that we may hear it and do it?’ 14 But the word is very near you, in your mouth and in your heart, that you may do it.
Mark head hand affixed = torah
Deut 6:8″You shall bind them as a sign on your hand and they shall be as frontals on your forehead.
“You shall therefore impress these words of mine on your heart and on your soul; and you shall bind them as a sign on your hand, and they shall be as frontals on your forehead.
“And it shall serve as a sign to you on your hand, and as a reminder on your forehead, that the law of the LORD may be in your mouth; for with a powerful hand the LORD brought you out of Egypt.
“So it shall serve as a sign on your hand and as phylacteries on your forehead, for with a powerful hand the LORD brought us out of Egypt.”
Revelation 22:4 (LEB)
4 and they will see his face, and his name will be on their foreheads.
Revelation 14:1 (LEB)
1 And I looked, and behold, the Lamb standing on Mount Zion, and with him one hundred forty-four thousand who had his name and the name of his Father written on their foreheads.
Context dictates it means something related to the torah and is man-made “it is the number of a man”
Rev Chapter 13
16 He causes all, both small and great, rich and poor, free and slave, to receive a mark on their right hand or on their foreheads, 17 and that no one may buy or sell except one who has the mark or the name of the beast, or the number of his name.
18 Here is wisdom. Let him who has understanding calculate the number of the beast, for it is the number of a man: His number is 666.
14:1 Then I looked, and behold, a Lamb standing on Mount Zion, and with Him one hundred and forty-four thousand, having His Father’s name written on their foreheads
It also contrasts this with those who keep the Torah:
9 Then a third angel followed them, saying with a loud voice, “If anyone worships the beast and his image, and receives his mark on his forehead or on his hand, 10 he himself shall also drink of the wine of the wrath of God, which is poured out full strength into the cup of His indignation. He shall be tormented with fire and brimstone in the presence of the holy angels and in the presence of the Lamb. 11 And the smoke of their torment ascends forever and ever; and they have no rest day or night, who worship the beast and his image, and whoever receives the mark of his name.”
12 Here is the patience of the saints; here are those who keep the commandments of God and the faith of Jesus.
(Rev 14:9-12 NRSV)
” the letter Chi used to abbreviate the name of Christ . . .”
“The letter Ξ means detached-from.
. . .
Ksustos (ξυστός) is a detached portion of a building, a covered colonnade, a walking place on the grounds of a private residence or gymnasium.”
(stigma has a value of 6 http://www.parthia.com/fonts/stigma.htm )
“The name, stigma (στίγμα), is originally a common Greek noun meaning “a mark, dot, puncture”, or generally “a sign”, from the verb στίζω (“to puncture”); the related but distinct word stigme (στιγμή) is the classical and post-classical word for “geometric point; punctuation mark”. Stigma was co-opted as a name specifically for the στ sign, evidently because of the acrophonic value of its initial st- as well as the analogy with the name of sigma.”
“Those Gnostic texts that discuss Jesus’ crucifixion and resurrection display a variety of views that, nevertheless, reveal some common themes. James is consoled by Jesus in the First Apocalypse of James: “Never have I suffered in any way, nor have I been distressed. And this people has done me no harm.”19 In the Second Treatise of the Great Seth, Jesus says, “I did not die in reality, but in appearance.” Those “in error and blindness….saw me; they punished me. It was another, their father, who drank the gall and vinegar; it was not I. They struck me with the reed; it was another, Simon, who bore the cross on his shoulder. I was rejoicing in the height over all….And I was laughing at their ignorance.”20”
The first people accused of antinomianism were found, apparently, in Gnosticism; various aberrant and licentious acts were ascribed to these by their orthodox enemies; we have few independent records of their actual teachings. In the Book of Revelation 2:6-15, the New Testament speaks of Nicolaitans, who are traditionally identified with a Gnostic sect, in terms that suggest the charge of antinomianism might be appropriate.
The seven heads of the beast are seven emperors. Five of them the Seer says are fallen. They are Augustus, Tiberius, Caligula, Claudius, and Nero. The year of Nero’s death is A.D. 68. The Seer goes on to say “One is”, namely Vespasian, A.D. 70-79. He is the sixth emperor. The seventh, we are told by the Seer, “is not yet come. But when he comes his reign will be short”. Titus is meant, who reigned but two years (79-81). The eighth emperor is Domitian (81-96). Of him the Seer has something very peculiar to say. He is identified with the beast. He is described as the one that “was and is not and shall come up out of the bottomless pit” (17:8). In verse 11 it is added: “And the beast which was and is not: the same also is the eighth, and is of the seven, and goeth into destruction”.
All this sounds like oracular language. But the clue to its solution is furnished by a popular belief largely spread at the time. The death of Nero had been witnessed by few. Chiefly in the East a notion had taken hold of the mind of the people that Nero was still alive. Gentiles, Jews, and Christians were under the illusion that he was hiding himself, and as was commonly thought, he had gone over to the Parthians, the most troublesome foes of the empire. From there they expected him to return at the head of a mighty army to avenge himself on his enemies. The existence of this fanciful belief is a well-attested historic fact. Tacitus speaks of it: “Achaia atque Asia falso exterrit velut Nero adventaret, vario super ejus exitu rumore eoque pluribus vivere eum fingentibus credentibusque” (Hist., II, 8). So also Dio Chrysostomus: kai nyn (about A.D. 100) eti pantes epithymousi zen oi de pleistoi kai oiontai (Orat., 21, 10; cf. Suetonius, “Vit. Caes.”; s.v. NERO and the SIBYLINE ORACLES). Thus the contemporaries of the Seer believed Nero to be alive and expected his return. The Seer either shared their belief or utilized it for his own purpose.
Nero had made a name for himself by his cruelty and licentiousness. The Christians in particular had reason to dread him. Under him the first persecution took place. The second occurred under Domitian. But unlike the previous one, it was not confined to Italy, but spread throughout the provinces. Many Christians were put to death, many were banished (Eusebius, Church History III.17-19). In this way the Seer was led to regard Domitian as a second Nero, “Nero redivivus”. Hence he described him as “the one that was, that is not, and that is to return”. Hence also he counts him as the eighth and at the same time makes him one of the preceding seven, viz. the fifth, Nero. The identification of the two emperors suggested itself all the more readily since even pagan authors called Domitian a second Nero (calvus Nero, Juvenal. IV, 38). The popular belief concerning Nero’s death and return seems to be referred to also in the passage (13:3): “And I saw one of its heads as it were slain to death: and its death’s wound was healed”. . .”
” . . . In ancient Greek and Hebrew, letters also represented numerals (as in Latin), their values assigned according to the order of the alphabet, alpha and aelph, for example, having the numerical value of 1. By adding these values, words could be represented as the sum of their numbers. This literation of numbers and numeration of letters was known as isopsephia by the Greeks and gematria by the Jews (which, in cabalistic practice, has been used to interpret Hebrew scripture). Suetonius relates an example of isopsephia when he records that graffiti appeared in both Greek and Latin lampooning Nero after he had his mother killed: “A calculation new. Nero his mother slew” (Life of Nero, XXXIX.2). In Greek, both “Nero” and “killed his own mother” have the same numerical value (1005).
If the Greek spelling of Nero Caesar (Neron Kaisar) is transliterated into Hebrew (nrwn qsr), the numerical equivalent is 666—although it should be remembered that this number was not represented as a figure but as letters of the alphabet or written in full. In other words, the “number of the beast” was not expressed as “666” (indeed, discrete Arabic numerals would not be invented for another five hundred years) but by the phrase hexakosioi hexekonta hex or the numerical values of the Greek letters themselves, chi (600), xi (60), and stigma (6).
But what is curious is not so much that 666 can be decoded to signify Nero but that the name is encoded in this particular number, especially since it could have been represented as readily in other ways. It only is when the words are transliterated from Greek into Hebrew and then calculated that the numeration adds up to 666 (nrwn qsr, 50 + 200 + 6 + 50 + 100 + 60 + 200). Even so, this is an alternate spelling, a letter being transliterated in “Neron” (nrwn instead of nrw) but not in “Caesar” (qsr instead of qysr). Although these forms do appear in the Talmud and an Aramaic scroll from Qumran, they no doubt complicated the solution to the puzzle.
For Watt, the significance of 666 is that its expression in Latin is the sequential Roman numerals DCLXVI, which parallels but is the antithesis of the “Alpha and Omega” that John uses to characterize both Christ (22:13) and God (1:8, 21:6). As the Deity represents the beginning and end, so the Antichrist is a reversal of the first and last, D (500) preceding I (1). To phrase this another way, 666 (or rather DCLXVI) signifies the Antichrist because that number signifies Nero, and Nero—who was a matricide, proclaimed his divinity on coins as the “Savior and Benefactor of the World,” and was the first emperor to persecute Christians—signifies the Antichrist.
If the Latin (rather than the Greek) spelling “Nero Caesar” is transliterated into Hebrew (nrw qsr), the final “n” in Neron being omitted (and its corresponding value of 50), the name computes as 616, which is the number indicated in the oldest surviving copy of the New Testament (the fragment illustrated below). If “Neron Caesar” is correct, it may be that the Latin was transcribed incorrectly, perhaps because the copyist realized that this transliteration did not equate to 666 and so omitted the letter, which changed the sum to 616. Still, each digit of 666 is one less than seven, the perfect number, and such mathematical play may have tended to establish 666, rather than 616. . .”