Where the boards sit in a pre-Xenon Bally backbox
Facing the backbox: MPU upper left (battery along its lower edge), voltage regulator/solenoid driver upper right, lamp driver lower left, sound board lower right, and the transformer with the rectifier board in the lower right corner.
Systems
Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part One, Clay Harrell (pinrepair.com)