My first Model 52-2 is an A774xxx serial number and it does indeed have the "0" stamp on the bottom leading edge of the magazine well! I hope that means you built this pistol of mine.

I have two 52's currently and this one is mint, I've only put about 600 rounds through this one, preferring to "keep it in reserve" while I shoot the wheels off my later model.

I've got a pair of 745's as well, I wonder if you built those also.

Thanks for your posts and sharing some history with us!