My humble thoughts:
1) Just in front of the forcing cone threads, on the frame, there's a sub inspectors stamp "v" in the proper area. The "v" has the little bend on the top of the left line. On the other hand, there should also be the same stamp on the back end of the barrel's flat surface.
2) The distance between the barrel and the loading lever is too wide. As Bishop said about the "step", it seems like a .36 barrel, which is a bit slimmer (don't know if the threads are the same).
3) The pinched front sight is low, which would be period.
1) and 2) speak against, 3) for
It would be nice if we got some measurements, from a ruler on the glas of the cabinet, in order to deduct lengths and diameters .....