mountainman

The CHI-RHO Symbol has often been regarded as a symbol
of ancient christianity.

However, what was the abbreviated standard of the roman
army role Imperitor Rex (eg: Imperitor Constantine Rex).
Did this look like "IPX" and usually written with a single vertical line
and the X and the loop of the P.

Has someone studied the evolution of citations
to this specific symbol?

Toto

There's a lot of previous discussion on Chi Rho, some of which indicates that it was a pre-Christian symbol. E.g. -

[Greek] X + P equals "Christ"? has a citation to Jack Finegan's The Archeology of the New Testament: The Life of Jesus and the Beginning of the Early Church, Revised Edition (or via: amazon.co.uk). You can search this on Amazon, and it contains a wealth of material.