On the RH side of the barrel at the breech they say:
William Briggs
Norristown, PA
Patented Aug 13, 1859 XX (serial #)
There were about 50 of these solid iron underhammers made by him. He used surplus musket barrels purchased from the govt. The barrel has a breechplug with a square tang that slip fits into the wrist and is secured with a tapered pin. The only two sizes I have seen are 12g and 15g.
The stock geometry is definitely odd. Definitely not something that has a natural feel when shouldered.
Most of the weight is in the last couple inches of the buttstock, so it handles kinda odd, too. This makes the muzzle very light, so it does point well.
Having only three moving parts makes it super simple to run and repair if needed.
I haven't got around to shooting mine yet, but it is fully functional and ready to be patterned. Number 27, .675" bore, 41" barrel.