It does have the two triggers which I have seen before on the older guns.
With set triggers, it's almost certainly a Great Plains model, although current GP's have 32" barrels and steel butt plates. (Could be wrong about the butt plate.)
The Lyman Trade Rifle has a brass butt plate, but never had set triggers.
Great Plains rifles are usually 1 in 60" twist round ball shooters. The GP "Hunter" model has a 1 in 32" twist suitable for conicals (and maybe sabots). You can buy 32" hunter barrels for current guns, but I have no idea if they would fit your older model. You could probably find out from Lyman customer service (1-800-22LYMAN) or Dixon Muzzleloading in Kempton, PA (Lyman's warrenty repair shop) at 610-756-6271.