I think Peter may be on to something when he asks if the frame is beefier where the S/N should be. I have 4 Inglis No.2 BHPs, and looking at the first picture I notice that there is an unfamiliar contour around the original proof mark above and behind the trigger, and where the cam fits through the frame. I imagine it is possible that the frame above the proof mark was welded up to repair a crack rather than replace the frame. The obvious question is who did it if it was done, given that it is unlikely that it was done at Enfield or during its military service time.