What a wonderful piece of history. It has all the distinguishing features of a middle period officer's fusil (c 1770-1780s). The flat lock & acorn trigger guard give it way, but the sideplate, the two private Tower proofs and nose band were also typical during this period for private contract officer's fusils. The muzzle pipe is an altered trumpet pipe that was also used on these fusils. It would seem that somewhere, there would be a contractor's mark or name associated with this piece? The real puzzle is in the engraving on the buttplate and sideplate. As Art mentions it looks to be of a different hand and not up to the same standard of the border, but at the same time, at least on the sideplate, as if a space was left for the standards??? As was mentioned, for the 23rd regiment, I would expect the Ich Dien - Prince of Wales feathers on the barrel. These were executed by the regiment's own contractors in a finer hand the the standards. More on the level of the border and shading on the sidepiece and trigger guard.
Another twist is the prevailing attitude toward officer's carrying fusils during the Revolution. 'North American' (ie F&IW) generals such as Howe, were in favor of their officer's carrying their own fusils and adding to the barrage. Classically trained, 'Continental' generals such as Burgoyne, Clinton and Cornwallis were of the opinion that officer's were to lead, not fight and discourage the use of fusils.
Facinating, authentic piece of history in great condition, that probably did play some role in the Revolution, even with these questions. Wonderful. Thanks for sharing.