With the semi-circle side frame reinforcement, I would 1st guess F.W. Keler but upon closer inspection along with the Sauer & Sohn tubes/tubeset, it was finished in Zella-Mehlis. So the components were sourced from Suhl and the overhanging scear A&D boxlock was completed and proofed in Zella-Mehlis post 1911. I wonder why he didn't show the flats with the date code? Anyway after the lower serial number there looks to be a HS monogram. I'd say the MM stamp is that of a mechanic at Sauer or a subcontractor.

Kind Regards,
Raimey
rse