Yes I'm very envious of you having that lovely big psu!
There's nothing I can see that's a real giveaway as far as changing the voltage is concerned (though it is pretty!) - you are going to have to investigate with a multimeter (and if you have one, oscilloscope).
Nothing special (chips-wise) on the small board - looks like it probably is to do with the battery backup and/or auxiliary functions.
Ok I've spent quite a while looking at this now, also a bit of googling. I think you have an SCR psu here, which is how switch-mode worked before mosfets started to be widely used for them. They were popular around the time your psu was built. That is why there are 2 big SCR's in there. I suspect the 2 T03 can transistors are driving a transformer (or maybe a pair of them) which provides the gate voltage to the SCR's - they are rated at quite a high voltage so a transformer makes sense here.
I guess that the big black transformer is the main high frequency power transformer. If you trace the wiring I would hope you find some big beefy rectifiers connected to it which give your output voltage, and the two SCR's connected (perhaps not directly) to the primary. I think the silver transformer is providing power from the mains to the switching circuits via a linear regulator. I think the big blue capacitors are providing smoothed DC to the SCR's. It would be helpful to know what voltage rating they are however to work out if they are connected to mains voltage (again, it would be better if you can trace the actual circuit).
I'm wondering if SK5 down by the board you pulled off can be jumpered, or have something connected across it to change the voltage. Again, wild speculation here.
There should be some kind of error detection circuit which monitors the output. It will compare this to a reference and produce some kind of signal (probably a DC voltage) which changes the frequency(?) (in a modern smp it would be the duty cycle, but this is old so it may be frequency). So I would try to work out what is creating the reference voltage, and change that.
Sorry this is all speculation, nothing definitive, but I hope it's got you a bit nearer. And I completely agree, 48-12v converters are for WUSSES!
I don't think I'm going to be able to offer much more insight based on photos - maybe someone else can. I'll chip in if I think of anything else.
This might be of some help:
https://www.repairfaq.org/sam/smpsfaq.htm
Also, if you put "scr psu schematic" into google you get some stuff that might give you some insights too, especially if you go on images.
This might help:
https://www.electronicspoint.com/scr-regulated-power-supply-t27850.html
As an afterthought, those big transistors seem very hefty indeed - adequate to drive the main supply, so it's a fair guess that they are not your gate drivers but perhaps work in tandem with the SCR's somehow....???