This looks like a very early rook rifle, with strong hints of Belgian (action and barrel )manufacture. The calibre could be .320 Revolver, the parent cartridge of the .300/ .295 Rook. With the early fore-end fastener and curly underlever, it probably dates from about 1870. The H&H serial number records for rook rifles are not available, and I would have to dig out the books to check the address details. Interesting though!