I have seen some strange wiring - Yes, I understand - the Ball-Index-Relay shall pull-in as the player makes points on a new ball - and the relay shall steady pull so when this ball is lost: The "brain of the pin" says "I am entitled to step to NEXT ball or next player or Game-Over".
I looked it up in ipdb (four player games) - up to "Pat Hand, May 1975" the pins have the wiring I understand - see JPG-1. To "Valencia, July 1975" the ipdb has no schematics. Starting with "Little Chief, Sept 1975" the wiring has changed, I show "Space Mission***, see JPG-2 - I do not really understand what the "position of the Bonus-Unit" has to do with "activating the Ball-Index-Relay".
Can You enlighten me ?, greetings Rolf
P.S.: Space Mission***: I stumbled over this strange wiring because of this topic: https://pinside.com/pinball/forum/topic/williams-space-mission-no-ball-advance-at-23-bonus
Pat Hand is still "old fashioned" - every new ball given starts with ZERO Bonus on the Bonus-Ladder. Little Chief and Space Mission and later Pins start (more modern) with the feature (I do not really like) "every new ball starts with ONE Bonus given for free on the Bonus-Ladder" --- maybe, maybe this "strange wiring for to activate the Ball-Index-Relay" has to do with this new feature ???
I tried it on my Space Mission - I made an Extraball - it is simply given with one bonus for free.