The guns commonly referred to as NW guns, Indian Trade Guns, almost invariably had serpent sideplates.
But fur trade companies sold other sorts of guns, in addition to the serpent sideplate guns.
Often these would have the same bore size as NW guns, which made a lot of sense.
I recall seeing a Parker Field double flint 24 bore gun. A late flintlock. It could easily have been one of the fancy guns that the HBC sold. The HBC Imperial series of shotguns, both muzzle and breechloading are well known.
Also keep in mind that there was an enormous export market for British and Belgian gunmakers into North America. Untold thousands of utility grade guns arrived in the 17th, 18th and 19th centuries. In parts of Canada, ml guns were still in use well into the 20th century.
