This is personally, one of my favorite symbols, and one of the favorite cards I did.
Chi Rho is Greek for the first two letters of Christ.
Chi = C
the P shape is the Greek symbol for Chi
the cross or X shape slightly turned is the Greek symbol for Rho
In the book it says this was an early version of the now well known Circle Celtic Cross. It was derived from the Chi Rho symbols. Even if all you do is just look at the pictures in these links you can see there is alot of variation and interpretation of the Celtic Cross or the Circle Cross.