I'm pretty much certain now that a combination of "J" and "&Co" with "Manton" is a giveaway of a... erm... creative marketing. Joseph Manton's gun could be a breechloader only if someone converted it. John Manton's shop had "& Son" (and Dover Street address. Manton & Co Calcutta went without "J" (and had Clacutta or London and Calcutta too).

However, these guns are a piece of history and interesting in their own right. Can you take a pic of the proof marks?

Russian Hunting Shotguns