Frankly, to my eye, the buttplate, guard and sideplate and likely lock look like they came off of a rifle very similar to the first of the 'Hachen' or Haga or whatever type rifles George pictured in RCA1. 21 or 22? Shorter barrel with fluted buttplate, percussion conversion (at the time). I don't have the book in front of me. But they don't look like late parts to me, they look pretty darned early. I'm sure the side edges of the buttplate comb were filed down in the restocking, and probably the edges at the rear of the butt portion too.
I would love to be able to get parts like that without the hassle of making patterns!