I have a Spirit of 76 that came wired sorta like that. They took the "Add Replay" signal and used it to trigger the Extra Ball Feature. It was easier to do, obviously, because S76 already has the Extra Ball feature and they just added another trigger.
On your machine, they've either added a relay to cut off the "Add Ball Count" signal and/or are using the Credit Unit in some creative way. Sounds like a fun project to figure out what they did! Let us know if you want help figuring it out.
I'm pretty positive that the schematic will still be accurate regarding the target lights. Pop bumper scoring/lights are controlled by the same unit as the target scoring and lights - the AS type, "I" Relay. I'd think that logic will be unchanged.