Westbury,
Do you think it had been converted to percussion, and then converted back??
I believe it was percussion altered in the early 1850's probably at one of the Federal Arsenals. I state because of the alphanumeric stamps "A21" on the lock, top of the barrel, in the wood to the rear of the breechplug tang, and buttplate tang near the "US".
In the 1851-1853 time period many thousands of M1816 Flintlock Muskets were percussion altered at both National Armories and Federal Arsenals at Watertown, Watervliet, Allegheny, Washington, St Louis, North Carolina, Frankford, & Kennebec. All of these facilities had a marking system, ususally alphanumeric, that was unique to each Arsenal or National Armory. Just at Springfield 86,565 M1816 and 26,841 M1840 muskets were altered. I would estimate that 99% of all Springfield M1816 and M1840 Muskets were altered. These were all brand new unissued muskets manufactured since 1832.
Springfield Type III M1816 and M1840 Flintlock muskets in their original flintlock configuration are far scarcer that original condition Kentucky Golden Age Rifles.
Check out Pete Schmidt's book U.S. Military Flintlock Muskets Vol 2 to confirm what I've written above.