Gibby,
I'm going to be a total @ss on this one. I think the side facing has been slabbed and a non-original lock mounted on this rifle. Three reasons for leaning this way: 1) too much dark color around the edges of the side facing, and more importantly, 2) the mortise edges around the lock plate are simply too perfect, too clean, to be as old as the gun. As you and I know, locks went in and out of guns over the years for repair and clearning, and mortise edges always got a bit nicked and dinged, especially on the back end where they were occasionally pried out. The mortise edges just look way to perfect, in my opinion, and 3) the flash hole is way off for this lock, or at least it looks like that in the last image.
The lock itself has ovality remaining in its tail and hasn't gone to a really rounded tail yet, so the 1820 date you first mentioned looks good to me... for the lock, but not necessarily the gun.
Shelby Gallien