First ball relay will be tripped by a switch on one of the point relays in head. This is accomplished by first score of ball one. This is what lets the player unit advance. If you ever notice on a game of this era if you do not score on ball 1 the ball does not advance. Some games the switch is on the 10 point relay, some games the 100 point relay. Check those switches first.
Last ball relay is tripped by the players unit in head. Since it is as multiplayer switches on the 2, 3, and four player are involved as well as score motor switches. Its best to get a schematic and trace the path to check the switches used.