Tom E Gun,
I suspect you will find that the large circle above the proof mark on the frame is the locking shoulder for the barrel. If you strip the pistol down, there should be a corresponding oblong circle on the left side under the slide lock lever, and will be the other side of the (rivited through the frame) locking shoulder. This was/is a fairly critical inspection point for weapons techs, as a loose locking shoulder could lead to signifigant frame problems. Inspection involved stripping the pistol, then lightly rotating the locking shoulder with round nose pliers to check for play, however oil displacement around the riveted sides was not a criteria to determine looseness. If found to be loose, and it was fairly obvious if it was loose, the pistol was to be backloaded for repair to 3rd line. If I recall correctly, we had so many unused pistols in supply we would simply backload the entire weapon. I also recall trying a small open end wrench for the inspection, rather than risk damage with pliers. Sorry, I cannot remember what size. It's been awhile since I had to inspect 25 or so in a weapons lockup.
