Check out this article written by a physics professor in 1991

here the professor expounds on why it is six hundred sixty six not 666. He uses various sources and textual criticism. He also uses iraneus as a source.

to keep in mind, the proffesor at the end in his notes debates the importance of keeping a hexadecimal based system in mind as being used by the beast. which is a sixteen based system, the equivalent of six hundred and sixty six being 29A in hex. i wont explain all of it, the majority is in the article below.

[link to www.answering-islam.org]

And this one by a seventh day Adventist rebuking another seventh day adventist for mistakingly understanding the number of the beast to be 6-6-6 when it really is six hundered threescore and six.

to explain this he shows the earliest portion of revelation we have left from the third century. The chester beatty P47.
Thank God that that one page happens to have revelation 13 18 in its oldest known form. And although it is xi chi stigma just like the one in the codex vaticanus, it is an earlier form of greek and does not look like the arabic bismallah.
also in this article he presents codex alexandrius and sinaticus which show them writing out six hundred and sixty six in greek majuscules or uniclas... the point is they didnt appear as walid shoebat saw them on the codex vaticanus.

Codex Vaticanus is an incomplete manusrcipt and was missing the book of revelation... a 15 century manuscript was added to complete the codex vaticanus, where it appears different from the earliest manuscripts of revelation.
[link to biblelight.net]

at last all post a blog post where someone debunks shoebats claims with similar reasoning and sources.

[link to eutychusnerd.blogspot.com]