The Caswells (Eli, John and James) were prolific makers of fine quality rifles in New York state.
Their first shop was in Lansingburgh (possibly 2 shops) and later in Albany.
Looks like your rifle was originally built as a flintlock and converted by someone else to percussion. The Caswells built high quality rifles with patent or forged breeches instead of drums which were lower quality.
Fine rifle you have there.
Your rifle was shot from a plank and could be called a plank rifle.