Thank you!
I don't think copper was common on original mountain rifles. It is more common on contemporary mountain rifles like mine and the few builders I know personally and aspire to be more like one day. I'm referring to Jack Duprey and Don Burton for example.
As Daniel said, but it's not an oddity, either (for thimbles, I mean). Some schools used it. Others didn't. Unfortunately, most of the books that show mountain rifles don't show closeups of the thimbles. And since copper tarnishes brown, it can be difficult to recognize it in poor resolution full-length photos of rifles.
(See this one, for example:
https://www.icollector.com/item.aspx?i=26475181 )
Here are photos of the entry pipe (iron) and middle thimble (copper) on an antique North Carolina SMR. Both the middle and front pipes are copper. I don't think there is any reason to assume they are replacements, but if they are replacements, they were replacements installed during the working life of the gun.

