while it may be possible to fit a bushing and new firing pin to that early Huntsman frame you have, as has been said it is not practical nor reasonable. I did purchase a frame like that from a member here who described it truthfully as a Huntsman frame because it said that on the side, however I mistakenly believed it was what we all currently think of as a Huntsman frame, a typical SB1 frame with a regular firing pin and capable of accepting and using shotgun barrels, it was not. Thankfully the member who sold it to me as a "Huntsman" frame was good enough to accept it's return and refund me the sales price so all I was out was the shipping.
It is not possible to use those early frames for any self contained cartridge, either centerfire or rimfire and I believe they can be bought or sold just as any other muzzleloader or sidekick with no FFL involved. They will not fire a cartridge from any centerfire or rimfire barrel that you may get to fit onto the frame and are incapable of detonating a modern primer or primed and/or loaded cartridge. They are manufactured solely to be capable of detonating a percussion cap if properly mounted and fitted to the appropriate barrel.
Sorry to say buddy you have a paperweight there and not much else. I do not know how long you have had it but anyone that buys or offers a Hunstman frame here should be careful they are not misled to believe it is a true SB1 frame and not an early ML only frame.
The frames from contemporary Huntsman's are SB1 frames capable or detonating centerfire cases with primers, what you have cannot accomplish that in any way....<><....
