Originally Posted by ellenbr
I would hazard a guess that it was an early, possibly pre-1893, long arm and the platform was altered in some way and passed thru the Z-M Proof Facility in the late 1930s.

Thanks. The gun does have a replacement hammer on the left-hand lock. Is it possible that when it was taken to a gun maker to have this fitted, was found to be un-proofed, and duly sent to the proof house? I imagine re-proof would not have been required on an already-proofed gun, but perhaps that was the point where legislation caught up with this one?