This might normally belong in the vintage computing sub-forum, but I thought it would probably be of more general electronics interest.
My teardown of a Power Supply Assembly, one of several used in an IBM 3031 mainframe circa 1978.
Supply input is 3 Phase 208V at 415Hz.
This is the larger of the two units I have, the teardown is of the slightly smaller one which provides 2.3V at 250A.
Primary regulation is via 6 SCRs on the secondary windings, followed by what I presume is a transistor shunt regulator.
The IBM 3031 Central Processor
Overall schematic
I surmise that this is a 3phase bridge rectifier for internal power, and a final output shunt regulator.
My Teardown:-