Thanks, Ken. As you likely noticed, the only area of the barrels where the pattern is readily visible is around the proof marks, although I can just make out a parallel pattern on the rib just in front of the breech, similar to the pattern in your photo. At the moment I am leaning toward the "twist" construction, particularly the "Birmingham stub" as per DocDrew.
I suspect that the gun may have been produced earlier in the period rather than later, as the forend is attached with an older-style wedge (or key) reminiscent of the percussion era. As I understand it, other methods of attaching the forend evolved in the early 1870's, and this style faded out.
As far as shooting it goes, the action is a little loose (or, more precisely, a bit off face) and the bores are pitted a bit more than I would be comfortable with in any of my modern firearms, although I have a Remington Model 6 .22 with a truly horrific-looking bore that shoots quite well. Still, it wouldn't hurt to have the piece evaluated. I'll have to see if there are any qualified smiths in the Albuquerque area - somehow I suspect they are rather thin on the ground out here.
