Hawg - you pretty much voiced my thoughts - "Enfield" seems to be evident and although I've seen a number of "keyed" barrels without "patent" breeches, one has to ask "why". But, on the other hand, if you're removing the barrel often and sticking it in a barrel of water to clean, the key saves a lot of wear and tear on the stock that knocking a pin out every time would give. Remove the tang screw, knock out the key and you're ready to clean by sticking the breech in a barrel of water and using a clean rod to pull it up into the barrel and push it out again - basically the same way shotguns are cleaned on the range down at Friendship. It has the appearance of so many of the "shotguns" or "fowlers" created out of surplus parts and surplus muskets that were sold by Bannermans and similar companies following the Civil War.
To the OP - don't get any of our comments wrong or take offense - if that is what this is, they are still a nice "collectible". When I was a kid, I used one of these fowlers that was made out of a cut down/sporterized Bridesburg musket to shoot clay pigeons with. The barrel was of course shortened and the rifling reamed out of it - .58 caliber so it was pretty much the same as a 16 gauge. $ were short so instead of using commercial wads, I used newspaper and I used the salvage shot out of old shotgun shells that I had accumulated.