FredC wrote on Nov 30^th, 2018 at 3:18pm:

Nice find! There was a US arsenal in California that began with a B, I wonder if that is their mark next to "MODEL". Not sure if I have ever seen that mark before.

Thank you. I was unbelievably lucky to get it as I had no idea how rare they were when I bought it.

I think the "BNP" is a Birmingham Proof House mark, from when it was imported. That's my best guess. Possibly brought over here in the latter half of the 20th Century?