The staurogram (P, crossed P) is derived from the Christogram and like the Christogram is a monogram comprising the first two letters of Christ in Greek, Chi (X) and Rho (P). The staurogram symbol is also called a Rho-cross. It is used as a control symbol on numerous late Roman issues. Justinian I issued a scarce nummus with the staurogram between alpha and omega as a "monogram" reverse type. Angels are sometimes depicted holding a staff topped with a Rho-cross. See Rho-Cross and Chi Rho.
Staurogram - 
Inverted staurogram - 
