Joe-
Might've had another set of barrels or been rebarreled into a shotgun.
http://dogsanddoubles.com/?p=281I looked again....I'm having trouble seeing it as ever a "pin fire".
Two things give it away as a pinfire action.
First thing is the fence detail.
Second thing is the very shallow section of the standing breeches.
Could definitely have either been a two-barrel set (and the present case either made later or it could have been delivered with two cases) or it could have been set up at some time with the shotgun tubes.
I'm fairly certain that the existing barrels were originally set up for pinfire ammunition too.
I'd a much better (in the hands would be best) look at the tops of the breech end of the barrels on the chamber-tops.
I expect to find dovetailed steel where the pinfire firing pin cuts were originally.
Cheers
Tinker