Going to bet those transistor base and emitter leads each are into a PCB mounted socket, probably a solder pin from a socket, used so changing a single transistor is only undo 2 screws and pull it out, then solder in a new Littlefuse to get the new one operational again.
The dual SCR is used with the LC filter to regulate the main DC bus voltage, using the conduction angle to control the voltage. Rough pre regulation before the pass module to reduce power dissipation, more because the dissipation without would need a bigger case, and another pass module with 2 entire dies of Motorola silicon, combined with a bigger external water tank and radiator, than any power saving. Going to bet the coolant circuit used a 50% propylene glycol and distilled water blend, along with a little blend of extra modifiers like sodium silicate and borax to buffer the pH to close to 7.00, and then there was another water to water heat exchanger that used a mini Sulzer tower to handle the actual cooling.
The white crud on all the metal is aluminium and zinc oxide, as this was used in a humid environment, with a lot of dissimilar metal contact, so yes the aluminium zinc alloy would corrode, wherever it was in contact with pure aluminium from the cover, as it would have a permanent electrochemical cell there, and the tiny gap would hold any condensed moisture in contact. not helped by the massive brass block either, which likely is driving all the reactions, seeing how shiny it still is except at the base where it contacts bare aluminium.
I doubt they expected it to last 30 years....