Hey, you guys are good at this. Lots of insightful info.
Morac’s lead on English proof marks led down an interesting rabbit hole.
As best I can tell, the BP proof marks on each cylinder chamber, frame and barrel are from Birmingham Proof House and possibly used from 1904 to 1925. The crossed pennants mark is a British military proof. The HB Proof Mark seems to also be from Birmingham, but either from 1957 or date unknown.
I agree it has the parts of a third model, and perhaps was originally mis-marked as BP in Birmingham.
If the mark with HB in it is from 1957, then I’m wondering if barrel was replaced, thus no IJ marks on top strap, and the proof mark was due to reproofing.
Fun mystery to try and solve.
Thanks for the knowledgeable help.
Got anything to add, please fire away.