As I think I stated, that's a very high serial number & with the 1/2 pipe side-frame reinforcement, it should really narrow the field. Sauer's serial number sequence was a little below that and Greifeldt, Merkel, Simson/BSW should have been about 1/10th that serial number. The contour of the 1/2 pipe side frame reinforcement doesn't mimic Greifeldt either but for sure it is some maker's Bockbuchsflinte(?).

Kind Regards,

Raimey
rse