No sign of a jones plug for coin adjustments. I think goodoldays is on to it. There is a yellow wire from the door to the w and P relays that's not on the schematic. It links those relays together making them both and the start relay pull in to give you 6 credits but only the P and start for 3 credits. Does this sound like I am on the right track.

Yep .. that sounds like it to me .

Agree the easiest option would be 3 plays for $2 , if you can somehow modify the slot to fit a $2 coin and fit a suitable coin mech .

or, being an EM what about 3 plays for $1 ?