I don't think that there is any doubt that it is a Belgian trade gun made from various French-influenced surplus parts. Joe, as you know, with these guns there could have been many sources for the used and unused parts used in it's construction. This sure puts them in the "head scratcher" category, doesn't it?
As pointed out, it does appear that the stock was for a longer gun, the band spring and barrel band inlets are apparent.
The Lock has a non-inclined brass pan exactly like the French M1816 flint musket, a change from the fence-less inclined pan of the French M1777. This lock could have been from a Russian musket contract by a Belgian maker or a Russian musket acquired by the Belgian trade from captured weapons from the Crimean War or....? The cock or hammer is of French/Belgian style but not exactly in form like either of those.
The trigger guard appears to be that of a Belgian Cavaleriekarabijn Mod.1816-22 as seen here:
http://www.abl1914.be/cavkarab181622gew/cavkarab181622gew.htm
While that page does not show a good top view of the two piece brass/steel trigger guard, I have little doubt that in form it is the same or at least similar as the guard seen on Weston's gun. French triggerguards the late flint and percussion period were of two-piece production and, depending on the model, of either steel or brass and steel like this one.
The barrel would be hard to identify with the information available, we can not see proof marks but certainly there are some somewhere, the Belgians, even at the time they were producing these "trade guns", by law had to proof their barrels. An interesting name on the barrel, I believe that Supercracker is correct, the name is "Sabria", a Middle Eastern woman's name. While this probably doesn't doesn't have any bearing on where the gun was marketed it follows a practice by some Belgian makers of naming the various products for record keeping.
The one part that I see on this gun that may not be a military surplus part is the front band. While similar to the military product it is very crudely filed and finished when compared to the similar band seen on all French and Belgian military arms. It is possible that it was a surplus rough casting finish filed by and apprentice for use on this gun where finish of appearance was not as critical as on a military arm.