Montana, I guess the proofs could be 1855-75, so not alot of help there. The locks, if original, are later than 1865 or so.
Given the Birmingham address on the rib, I assume the gun was not marketed in the U S with only the Henry Buckley name being the U S part. I have looked at a few Mullins since your post and don't see any John or Patrick Mullin guns, muzzle or breechloaders, that are "carved" in this style. I would love to have Bill McPhail chime in.

http://www.bakercollectors.com/