That's quite a beauty of the rifle, with a good honest history to boot. I can only hope to own something like that someday.
In the mean time, for those guys not all that familiar with swivel breech works, notice the difference in lock style with this one and the other one, both ahead of the swivel joint, and the shape of the lock plate behind the hammer. This is a hand made lock, and this type of lock was used on all the flintlock swivels. Maybe not exactly the same, but the same in general and hand made.
On this rifle, the piece hanging down ahead of the trigger guard is the frizzen end from the barrel that is in the lower position now. The barrel lockup is released by pulling back on the forward part of the trigger guard.
On the other gun, the lock is a commercially made part designed early in the percussion period. 1830ish. As far as I know, this style of lock was never made originally, in the US, as a flintlock.
The bit hanging down ahead of the swivel joint is the barrel lockup release, and it is either pulled back, or pushed forward, to release the barrels for rotating.
John