Yeah, the stamp doesn't look like a BP, but I'm pretty sure it is. The marks on the Rudd as well. Take a look at this image. The first proof mark in the third row from the top is the Birmingham black powder provisional proof. It looks more like an S when deeply struck. If you take a look at pictures of barrel flats from that time period on the net you'll see some that give a clearer image of the stamp. I could be wrong though. I've got 2-3 dozen British guns from that time period with those marks. The BP provisional proofs on some are clear, and others look like an S or are pretty much unreadable.
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