FWIW, below are photos of a Dauphin County John Shell barrel. It's a 42.5", .40 caliber smoothbore with a very slight (1/16th") straight taper. Only three barrel lugs.
This is the breech barrel lug, stapled not dovetailed, but clearly slotted:
This is the missing muzzle lug. A bit of an oops! on the pin placement. Makes me feel better about my work. Or, could be a very thin forestock.