1920 Single date

I am not suggesting that the pistol was re-worked from a dated WWI military, but rather was new made (perhaps commercial hence the proofs) in the later 1920's. If this pistol found it's way into the Navy inventory (say 1928-33) it would have been entirely correct to have the "1920" PROPERTY marking added to the blank chamber. As I'm sure you realize that the 1920 marking was used long after the year the order came out to mark these weapons (1920). The Navy started using 4 inch. barrel P.08's right after WWI and continued this until 1945. Many and perhaps most of the 6 inch. Navys still on hand in the 20's were converted (re-worked) into 4 inch. versions. The O followed by several numbers ia a well known and documented as a Navy property marking and only that. I can not comment with hard evidence on the Mauser Re-work theory, but I would strongly recommend that you take a look at Still's book, Weimar Lugers, as he provides some decent information on these pistols and the German Nays's re-work program.