It looks like it might have been a rifle that was bored out smooth. It has a rifle front sight. Also look closely at the muzzle, it looks like the left muzzle has been completely bored through where it touches the right barrel. It might have coned muzzles though, it's hard to tell from the pic. The rifling might start a couple inches back. Inspect it in person if you can.

Beautiful gun, but it may be un-shootable which will affect it's value even if you have no plans of ever firing it.

“I left long before daylight, alone but not lonely.”~Gordon Macquarrie