[HerrKaiser]

Then only semi-plausible idea I can come up with is that the pistol simply retained all its military proof markings in lieu of commercial nitro proofs as it was a cottage industry commercial rework post WW1? My post-war commercial's chamber has indications of a date having been scrubbed from it as it was reworked for commercial sale. But it was given nitro proofs and a commercial style SN.