Hi Granville,
I think you have a civilian sporting gun from the late 18th century. It looks British or perhaps New England with imported or reused British parts. The consensus among at least British and American firearms historians is that officer fusils were not common, not government issue (meaning they were purchased privately), always had sling swivels, and were set up to mount full sized bayonets. It is important to understand that both British and American militaries vacillated about officers carrying firelocks. Some senior officers believed no line officers should be focused on loading and shooting, rather only on leading their men, while others thought officers needed firearms to protect themselves.
dave