Great video! I'm proud to be the one who fixed the schematics from my bad copy. Very glad to see that someone else had a use for it. It actually took many, many hours to draw those lines!
I had the exactly same problem, bad switches, and yes, the ohms and voltage switches were the worst. But, you actually can apply some contact spray on the ends (opposing ends from the front) and you also can take the switches apart without desoldering. They are a mess though. The pins, as you said, go across and the wipers contacts the pins directly. If you remove the front, then the metal bar in the front of the switches, the bar holding the wipers can be removed. The wipers are just loose metal tongues and will fall off when the wiper holding bar is removed. It's not trivial to put it all together again, but this is what I was forced to do to fix those intermittent problems.
After the fix, the instrument worked again for many years, until this very summer (northern hemisphere) , when the instrument suddenly fell out of calibration, showing about 20% to high values on all voltage ranges. This is outside the possible calibration ranges of the trimpots. I suspect it
may be due to the summer here in Sweden, which has been unusual wet with RHs approaching 80%, so I will wait to the autumn and try to recalibrate it.
The unpopulated socket was correctly identified in the video as a plug-in socket for a serial- or HPIB-interface, not for an IC. I actually made my own interface for it to my own specification and has therefore invested a lot in the instrument, why I also will go the extra mile to recalibrate it once more (even if it takes replacement of components).
I believe I have discussed the RS232 interface in another thread (the thread where I posted the schematics).
Again, great video! Great recognition factor!
Edit: And yes, all the same confusion. I also looked at the chopper circuits, discovered the two chip chipset, and also desoldered and checked all the pots. And, after the fix, made some modifications with my interface board. Negative input and system ground is the same potential, so it was possible to have a relay with a resistor to +5V and voliĆ” - free diode measurement range! (Control from RS232, not from front panel.)