Zedcat,
Nice rifle, I believe the 1773 that you are anticipating to be a date is actually part of the engraving décor of the lock. I believe the hammer is also not original to the rifle, it appears to be European in nature. H Demuth was the lock maker and he (I believe) is from the States. The documented Demuth family members were from Lancaster and Bushkill. I am also guessing your rifle to be of Lehigh origin or North Eastern Berks County. My guess on the date is probably around 1819 / 1820. I am new to collecting so this is not an educated opinion.
Good luck
JLB