I think its completely legitimate, just not British. Though I can't see it very well, I may even have one of those trigger guards in my
junk" box from a wrecked musket I bought when I was in high school. Does it have a huge bore? My musket must have been .100 caliber.
That said, there is good reason to believe that large quantities of surplus foreign muskets were imported before the War of 1812. In fact, Congress remitted import duties on small arms yearly for a number of years to encourage their importation. The shortage of usable arms was critical and, with the new milita acts requiring everyone to be armed, there was a huge demand for cheap muskets. So... the War of 1812 part of the story does hang together. I think the gun itself is likely to be somewhat older — old enough to have been surplussed off after 1800 which suggests 1750–1770 and probably Dutch (in a very wide sense) or made in the low countries for one of the many minor German states.