No expert but did a little searching and found indeed, what Rich says. Suhl narrows it down somewhat. I don't think it is Merkel. I don't think Merkel is old enough.
Suhl - Wikipedia, the free encyclopedia
So, the 11.15 x 60 is an 11 mm black powder Mauser caliber? I am confused what 50s you speak of. The gun is too old for 1950s and too new for 1850s.
The proof marks show a big eagle, that's the Reichsadler.
German 1892 proof rules and tables - The DoubleGun BBS @ doublegunshop.com
Just looking at that document, I'd guess this thing was made in the 1890s. The Reichsadler was used in the Weimar Republik and Nazi Germany but looked different (more abstract) later.
I would search for Suhl gunmakers in that era and you should be able to narrow it down quickly. Also consider the possibility that it was modified from a drilling?
Keep us posted. Cool gun!
G