Remove the four screws on the side panel and it falls right off.

Exposing this beautiful piece of work inside. This appears to be the main voltage adjustment board, as it has multiple trim-pots on it

Everything on this board is top notch. Precision resistors from micro ohm(1%, 0.1%, 0.05%), larger resistors from Dale, capacitors from Sangamo and Sprague, Zener diodes and transistors from Motorola, potentiometer from Allen-Bradly, Vishay and Bourns, op-amps from Analog Devices. It's like a who's who of American electronic components companies.

The manual says the Op-Amps are from Sprague but they clearly have the AD logo on them.