What each System 3 A-number board is
A1 CPU; A2 +5 V power supply; A3 driver; A4 dot matrix display; A5 aux supply (+/-12 V for sound); A6 sound; A20 aux sound; A8 display controller (dot matrix or alphanumeric); A11 aux driver; A12 transformer panel; A13 playfield resistor board; A15 flipper sensor; A16 filter; A17 diode board; A18 ground board on the transformer frame; A25 opto interface; A26 game controls (test, tournament, volume); A27 communications adaptor.
Systems
Source: Repairing Gottlieb System 3 Electronic Pinball Games from 1989 to 1996, Clay Harrell (pinrepair.com)