There were quite a few ML to BL conversions done in the 1860s and 1870s, so it's possible I suppose. From what I can see from the pics, it doesn't look like one to me.

Pity about the right barrel. I don't know of anyone offering a relining service for rifle barrels. Boring out to a larger caliber, yes, but not relining back to original caliber. Sleeving to a comparably low-pressure black round is an option, but you'll end up with a black-barreled gun. Lovely gun.
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