James,
The bottom line of the stock does not "swell". It follows a same curve as the forearm lower line out to the entry pipe as you would expect. The sides of the stick swell out in a fashion similar to a brown bess, but no where near as pronounced as a bess stock does. The square tang is unusual on an English gun. Even a plain breeched English made gun. I wonder if the barrel was reworked and reproached for some reason, or if German styling was used because it was a rifled barrel.
It is hard to tell what may have happened in the life of a gun. I have a gun I bought from Holts a few years ago that appears to have started life in John Manton's shop as single barreled flintlock fowler in the late 1790's. At some point the middle of the gun from the front of the lock just at where the false breech would have been was replaced by a rotary lock breech loading mechanism. The rear half of the flintlock was retained and converted to a hammer sidelock. It's a really nice 220 year old 16 gauge breech loading shot gun woth a Manton twist barrel on it.
I guess if the barrel was good, they kept using it as long as they could, restock, rework, etc. and keep going.
Mike