That's the sweet part of White's system most parts are interchangeable like the breechplugs as long as you stay with #11 cap or musketcap or 209. I never saw a 209 BP and traded off the only musketcap BP I did own for a #11.

The shotgun BP's are bigger as they are the BG series plug in #11 cap.

The Whitetail, Bison, M97 and Thunder Shotgun all use the same hammer and spring.

Not sure if the Super 91's use the same hammer as a Thunderbolt, but the M98 looks like it would, but I haven't seen one of them either.