Quoted from jibmums:
Here are pix of both upper & lower switches. Right & left side were identical so I just took the two photos. I think I remember reading that the low voltage switch (with the smaller contacts) on the upper flippers triggers a "shot fired" sound effect, if so then that wouldn't necessarily need to be replaced, right?
No, don't replace the low voltage switch, just clean the contacts with a new $100 bill.
All you need to replace is the EOS Switch part. You can use a Williams one, or call PBR and get the exact Bally replacement.
Don't forget the plastic spacers are needed when you stack switches - you DON'T want to put high voltage down the switch bus.....