It could well be an owner's mark - added years later.
I've run across these before - mostly done in the 1950's and '60's, when it must've been popular among collectors.
(The most memorable was one done with a Maltese Cross, and if you think 'that' didn't suddenly make everything 'German' - then you have a stunted imagination.)
That collector marked every damned thing he could get his paws on with his stamp - including leather and wood.
Your best bet is to backtrack the recent ownership of the weapon through whomever may've owned it in the last few decades.
Good Luck!
Vaya,
Scouts Out!