It's an 80% Brown Bess kit. They were banned in the 1700's after Queen Cankles and Mothers Insistent on Acting Important lobbied parliament that the peasantry must not be allowed to have such liberties.
More photos. When i got this, did some research. The proof look d like sheffield or some outlying town. London, i read, was stamped on guns of the time, to add panache as the london made guns were thought best.
Reminds me of regret I carry from a flea market 10 years ago.. A guy had a real nice blunderbuss similar to yours but in much better shape and he wanted very little $$ for it I think a 100. It was old (not a repro) and clearly a conversion to percussion - but not a brass barrel (IIRC).. At the time, I was trying to reduce debt and simplify things ..and just walked away.. I always see cool stuff and deals when I am tightening the wallet.!! '