The firing pin is the odd shaped thing the hammer hits when the breechblock is closed.
There were 31000+ percussion Sharps converted to cartridge, a lot less expensive than a new rifle.
There are three main versions that I know of, plus some variants of interest mostly to collectors:
1. .52 rimfire - rare and valuable but unshootable, no ammo in a century.
2a .50-70 centerfire with original six groove barrel. The Army would accept any barrel up to .5225" for simple rechambering and new breechblock. The ones above .515" are not very accurate but adequate for shooting at Indians on horses.
2b .50-70 centerfire with barrel relined to three grooves, same as .50 Trapdoor.
A friend has one that is fun to shoot, the short carbine definitely rears up in recoil but the kick to the shoulder is not too bad. He was troubled by misfires until he found that the hammer was contacting the screw holding the cover over the old pellet primer compartment. He filed that flat and it shoots just fine. One of the Minute of Commanche models, though.