Nice unit, just needs a good switch clean, a new set of capacitors ( 50 year old electros are 30 years past the best before date) and a 220V transformer and I will happily use it. Nice KV divider bridge there. Only issue is that output voltage is only really accurate at close to zero current.
I would take the old caps out carefully, strip them apart and then open the case, clean out the old junk inside and place new modern units inside, as the case looks so nice. Values would change though, to 100uF 450V for the first one and 100uF 200V with a series 100R 1W resistor for the second one. The 100n cap to ground would be changed as well to a 100n 630V unit, along with the 1uF output capacitor being changed to a good 1uF 400V foil type. Circuit diagram shows a second cap to ground, seems to not be present in the unit.
If you wanted nmore accuracy the only way is to replace the zener diode with a series connected string of 6V8 diodes, about 16 in series adjusted to a zero TC current would make a low noise relatively stable replacement for the avalanche diode, giving 108V as a stable source.
As to a switch cleaner, first will be to go to Amazon and buy 2 bottles of Caig Deoxit and the Caig cleaner, and use them to clean the switches using a ear bud to wipe each contact. Then place a drop of light mineral oil ( not from a spray can) on each of the bushes and each of the pot bushes so that it can soak in. You might want to have a spray bottle of the Deoxit to spray into the pots as well to clean them, though you will have to move them from end to end after application and then run through the full calibration procedure afterwards to get the currents right. That will make each switch operate like new again, though you will have to look on the side of those loose knobs to find the small hex ( 1/64 IIRC) grub screw that locks the shaft and remove it, clean the shaft and then tighten it again. If the screws have cracked then a small bit of 2 part steel epoxy works well to lock it again rather permanently.
