They are the GM of pinball. whether it be chev or gmc.....they are all GM vehicles with the same parts. Just like all the wpc machines....williams or bally, same thing and same parts etc..No difference...just the name..
But they are all WMS games.
They made one williams and one bally then williams then bally. rinse and repeat.. Bally only parts were used up during the system 11 era..