[Hugo Borchardt]

Welcome. Nice mystery. Would love to see photos of the gun.

Reich eagle stamp on the tag makes it extremely likely to be a period placement.

Last 2 serial digits strongly suggest the tag is associated directly with the gun, so the tag is likely not something random hidden under the grips by the guns owner. Would be too much of a coincidence.

Claim tag for checking weapon in for repair? No reason for it to end up under grip. It could some type of "keep this with the gun in case or to prove" tag that the soldier put under the grip to comply.

The fact that someone bothered to place it under the grip at all is key. Repair inspection tag is indeed the best guess, as others likely would rarely or if all be encountered because they were discarded during subsequent routine gun cleaning. Assumption would be that the signature under the "59" is the inspectors illegible best guess "J. N. Tal".

It surely was hand torn from a perforated sheet of same, as it has 3 torn perforated edges, and a forth perforated edge on the right side which is not torn and you can see that the flat non-perforated right edge was the right edge of the entire bordered sheet.

I can't tell from the photo, is it about the size of 3 regular US postage stamps? I'm sure you would have noted if there was anything on the obverse?

In other words, Colonel Mustard did it in the study with the lead pipe.

Thanks for sharing!!!