I'm with Geo. It's a Reichswehr provisional mark. Probably 3rd Infantry Regt. weapon #36. I have seen a few with a 3rd Regt. provisional mark, but none had complete grip strap markings. It appears that this one did, but they were scrubbed.
It could be 3rd Company 36th Regiment. If so, it would explain why the grip marks were scrubbed. The 36th Regtiment would have been dissolved after 1921 and no longer existed.
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