DeWitt Bailey's "British Military Flintlock Rifles" has some late flint rifles made by Tatham as high grade gift guns for the NA natives about 1816. Its an excellent book containing chapters on rifles in America, Indian guns, Loyalist rifles etc with dates and footnotes ect.
The BEST idea is to buy the original Purdy Rifle plan from Track of the Wolf.
The stock design and cheekpiece are virtually the same as the late Manton/English flint guns. Just stretch the forend to take 2 keys. Even the lock internals are the same.
To make it right you need a true Manton/late English style lock from Blackleys or The Rifle Shoppe. The L&R #1700 is too early and the big rainproof L&R is too big.
I used the recessed breech Manton castings from TRS.
If I were doing it again I would use this or the V pan lock. TRS even sells breech castings for the lock shown.
Dan