This has been very educational. I have enjoyed and learned from all the feedback. After having looked at hundreds of pictures of percussion DBL BBL shotguns on the 'net, I'd like to share my 2 cents worth of speculation, and I may amend this entry after I visit the shotgun again today, so here goes...
I believe the shotgun to be in its original form - unconventional as it may be. I see that the guard is rifle as the space inside the trigger area seems cramped. And of all the pictures I've observed of DBL BBL percussion shotguns, none have this curved stock. However, this gun's stock has a thick, flat butt - typical for shotguns. I believe the maker was a rifle maker. And all he had in his shop were stocks and parts for rifles. I think one day he got a wild hair up his nose and decided he wanted to go pheasant, or duck hunting and made himself or a family member this shotgun. Or, one of his customers asked for a custom 'rifle stock' shotgun << which is within the realm possibility.
I'm going to visit the gun again today and have a closer look at the guard area and scrutinize the mortice to attempt to confirm whether the guard is original to the gun. I'll let you know what I think honestly as I have no prejudice one way or the other with this gun.