Encyclopedia of Trade Goods (Hanson and Harmon 2011) has a similar trade gun on page 231. Shares the same style sideplate, round barrel, and buttplate. But, lock is a little different and it does not have a rear sight like yours does. They say "The last flintlock model of British common gun for Indians, made by Robert Wheeler & Sons in the 1820s-30s. It has military style butt plate, side plate, butt stock, and a round barrel. The only North West gun features are the lock, binding strip at the muzzle, ramrod guides, and trigger guard. It has a 36 inch barrel, .65 caliber, and weighs 6 pounds."