There are two of the August Huhnstock made guns still extant. The one shown in De Witt Bailey's book on British Military Flintlock Rifles numbered 184 was sold by Morphy's in 2020.
https://auctions.morphyauctions.com...ANOVERIAN_PATTERN_1776_FLINTLO-LOT483833.aspxThere are some good photos on there that, combined with the measurements in Bailey's book, should allow a builder to build a suitable replica.
The rifling is very fast 1:27 on this example. It is also interesting that the barrel is pinned in place, and the pan is bridleless, the only real innovation is the rammer retainer system, which was removed on this example at some point, otherwise, it is a pretty plain German military type Jager rifle. The Baker rifle is closer in butt design to this rifle than it is to the more numerous Birmingham made 1776 rifles.
Mike