I have an interesting book by an ex-Muslim terrorist who has become a Christian. In his book he puts forth a theory on what glyphs looked like which the apostle John originally wrote in Revelation 13:18 for the mark of the beast. His theory is a visual one, in which you take the Greek abbreviation for 666 found in many of the manuscripts and turn the letters on their side. So take χξς and turn the letters leftward onto their sides, and it looks quite a lot like a famous slogan in Arabic script that is central to Islam.

I am quite sure the theory is not correct, but here are snips of the four earliest Greek manuscripts of this verse that we have:

The abbreviation χ̅ξ̅ϛ̅ for 666 in Papyrus 47:

The abbreviation χ̅̅ι̅ϛ̅ for 616 in Papyrus 115:

The number 666 written out in Codex Sinaiticus (ℵ) as εξακοσιαι εξηκοντα ἕξ

The number 666 written out in Codex Alexandrinus (A) as εξακοσιοι εξήκοντα ἕξ:

I have an updated footnote on this in my Revelation PDF. Download it here.