Very Old Text (Rev. 13:18-end) in glass display at Bob Jones University Library - Greenville, S.C. Greek text showing Rev. 13:18 "number name"	(stigma looks like an ear piercer-marking a slave-Christ Jesus Slave-says these three initials)
Chi,         Iota,    stigma - 616	Chi,         Xi,    stigma - 666
	Zeus or Iesus same person, same name, spelled in different centuries, as the letter form changed.

"I or Zi"  "Xi" the greek Capital Form in small form.

The Letter Z-...The Greeks changed the Semitic name zayin to zeta and used the sign in several forms...From one of the Greek forms the capital letter came into English unchanged. (From Compton's NewMedia)