This is the circuit that keeps track of the score. When MISSED goes low, that increments either score 1 or score 2, depending on whether L or R is active. (An inverter between L and R has been added here to ensure that they're not both active at the same time.) When one of the scores reaches 15, then STOP G is brought high, ending the game. There is also a switch which the arcade operator could have used to end the game at 11 instead of 15.

You can reset the score by bringing SRST high at the bottom left.

Of course there's nothing stopping you from continuing to increment the score past 11 or 15. This wouldn't be possible in the actual game because MISSED cannot go low in attract mode.

Next: Game Logic.