Oldest Manuscript P47 shows Σ Sigma not Ξ Samekh for 60 in the 666 from Revelation 13:18 

Oldest Manuscript P47

shows Σ Sigma
not Ξ Samekh
for 60 in the 666
from Revelation 13:18 
The shape (Σς) and alphabetic position of sigma
is derived from the Phoenician letter 𐤔 (shin) 𓐮 
The name "sigma" may continue that of Phoenician samekh 𐤎 𓊽 
the letter continued through Greek  xi,
represented as Ξ.
Ξ Xi is distinct from the letter χ chi ,
which gave its form to the Latin/English letter X
With all these formula's,
might as well just use 𓄤 Ι iota
to get the point 𐤉 across
about Samekhi
ΘTheos or I or the new J 

χΞϛ : Chi-Samekh-Stigma
χΣϛ : Chi-Sigma-Stigma
χωϛ : Chi-O-Stigma
χΙϛ : Chi-Ιota-Stigma
χxϛ : Chi-Xi-Stigma