Thanks, Stan & 300846 for the conformation on the GP. That is the way I have always heard it stated as well. I do have a copy of Wirnsberger's book, just had not gotten it out. If you look at the mark itself though it is rather easy to see how the two letters intertwine to form both the G & the P which as I stated in my post stand for Gunmaker's Proof.

What would a C represent?

Miller/TN
I Didn't Say Everything I Said, Yogi Berra