Hi Ash

A lovely little rifle, certainly made in the Birmingham trade and retailed by R Jones ( not listed in Brown's at that address)

64 bore is .417 land and .425 - 430 groove diameter

The proofs are Birmingham, 1855 - 1904, but I cannot see clearly enough to make it closer, probably the earlier end of that scale

My first thoughts were as Huvius's - too long for a rook rifle but as you say, 3" the .400 kynoch is logical, although according to Fleming and Hoyem, they were only at 2 3/4 " and 3 1/4"

An interestng conundrum!

Good luck in your shooting

TH44