I'd vote with Chief here about date, retailer and diet, though it looks like it's in pretty good shape and it might bring a bit more from the right person. I'll _guess_ that the DRGM (Deutsche Reichs Gebrauch Muster - not quite a patent, but a design registration) might apply to the design of the self-cocking (selbstspanner) mechanism. Typical 1892-1912 German proofs.
I've not seen first part of the style of mark "P16___65.9,75 S.P.28,5 BL", but the 9,75 S.P.28,5 BL looks more like a proof level load than a service load: 9,75 grams (150 grains, nearly 5-1/2 drams!) Schwartze Pulver (black powder) and 28,5 grams (439 grains, or just over an ounce) Blei (lead). The P16___65 is temptingly close to Patrone 16 (16 gauge cartridge) 65mm long.
