These proof marks are a bit of a witches brew & not always exactly to the recipe it seems, to me at least.
From the barrel flats I interpret the Diamond 12 C as the possibility of 2 3/4" chambers.
The crown over BP, definitive Birmingham proof 1904 onward.
The crown over BV, the view mark again.
Now the thing is that it has neither the black powder or the nitro powder definitive proof marks. ??
What sayest the more learned ?
Give you $ 400 for it.
O.M