You are I believe asking the same question. On a basic double rifle or SxS shotgun, the firing pins strike through holes drilled through the face of the action. Some better grade doubles have removable (threaded) discs through which the pin strikes. The discs allow easy access to the pins (or strikers as our Brit friends would say) for cleaning. This was more an issue back in the day when primers were less dependable and could and often did extrude copper back into the firing pin hole. I have never seen a cheap quality double with bushed pins, but many fine ones do not have them (I have three "best" London sidelocks and none have bushed pins - however a couple of my really good German guns do). The discs are turned using a spanner. Some have three holes - most only two - in which to insert the spanner and remove the disc. A cased, high end double with discs, would have included an ebony or perhaps ivory handled spanner for that purpose (along with screw-drivers exactly matched to the screws on the gun).
