Ah, the mm stamp & it being void of a datecode, which Zella-Mehlis officially commenced w/ in 1911. It somewhat resembles a Greifelt Modell Nr. 180 but the possibility exists he sourced E. Schmidt & Habermann. Yes, I would hazard a guess the scope mounts were added a bit later, possibly in Otto Bock's shop. The S in a chevron is the mark of the Schilling forge.

Cheers,

Raimey
rse