Turn it on, Dave! we want to see sparks
1200W - beauty. This must a pretty EFFICIENT power supply - those tiny fans and even the large metal case are not going to be able to shift much more that 100W of raw heat - maybe much less, even.
Looks like the build is 2001
This one DC-DC switcher board PCB is probably supposed to cover several different output combinations. I expect the "apparently short circuited" second output capacitor would have been a second series-wired output cap (like the input ones are) for the 600V version of this PSU (since most electrolytic caps top out at about 450V DC). But it simply isn't needed in this 300V model.
It looks pretty clean and dust free. Has it seen no service, or is it recently serviced, I wonder.
The cardboard is a strange choice, but at least it doesn't melt! Probably there to direct the air flow over the muscle parts.
The main DC-DC switcher may have to be a combination of buck/boost topologies, so that it can produce both over and under the incoming DC voltage.
Edit: Ah no - on second thoughts it is probably a flyback design - those are high frequency stepping transformers, near the output end (and that's where the input/output isolation comes in, of course).
There does seem to be a lot of "optional" control electronics, if that board is indeed optional. Like your own power supply design, Dave, no doubt the voltage and current feedback loops are analogue, and not relying on any of this fancy digital stuff.
Hmm, 5V TTL devices in the same box as 400V DC at several amps.... <shudder!>
Turn it on.