I think a lot depends on what languages John knew; and, if he was looking or thinking in Greek, what system of numbers the Greeks used at that time. (Right around that time, they switched from a Roman type to something more modern, letter for number, but I don't know the specifics.)
My theory is:
1) The "666" everyone talks about is meaningless as a number in this context. Those numerals were not in use in the 2nd century AD. At best, those symbols would only have been a decoration, and would not have meant anything.
2) If one accepts that John was relating a "vision", which I do, and that much of this can be taken literally, then the issue is to try to figure out what he "saw" in the vision.
Remember that we are talking about someone nearly 1800 years ago, having no benefit of hindsight with modern knowledge or terminology. Think how someone only 150 years ago, or even your own great-grandfather would have described a lot of the stuff we have around the house today. A television? Maybe a "picture brought to life?" (That's in Revalation, also.)
John, as a Greek, of course would have known that language. In the Roman world, even if he didn't have an extensive knowledge of Latin (he may or may not have), he certainly would have known enough to get around. He would have known for example, that DCLXVI was six-hundred-sixty-six. Then there would have been the Greek (Lambda-lamda-lamda ?? ) or the other system, as mentioned above. Could have been another language/alphabet, as well.
Given a vision, much of which he could not comprehend, he did his best to relate it. He saw something which meant 666 to him, but it could have meant someting else. As a previous poster said, it was the mark of a man. Therefore, the symbols identified the man, and could simply have been a name to us, or possibly a political party symbol (less likely, because it seems to indicate one man). Or, a comapny or other mark relating to the mechanism to conduct trade. In the hand? A chip perhaps, manufactured by some co. with that symbol. Forehead? Retinal scan, perhaps. Yes, I know where the eye is. But, how would the machine used to check glaucoma look to someone 1800 years ago? Or, as some have indicated, it be a tattoo relating to a political loyalty. Whatever it is, I'd say the answer lies in finding the numeric symbols for 666 in John's time. Ancient language scholars out there?