A few years ago I purchased a pair of differential voltmeters because I was looking for aluminum equipment cabinets and thought these might work. The front panels were actually a full 1/4-inch think, which was a little more than I bargained for. These were military-grade equipment, and I think both of these items had been in use by the Army.
I got them from Fair Radio Sales “as-is.” The shipping cost almost as much as the equipment did. These meters were made by John Fluke Mfg Co, Inc, of Seattle in the early 1960s. They used mostly vacuum tubes and various other technologies now considered Legacy. The idea behind a differential voltmeter is that you compare a known voltage with an unknown one, and use a meter to tell when they are equal. The settings you used to get the known voltage are then equal to the voltage you wanted to measure. Ponderous. Today’s digital voltmeters do the same thing, except they “turn the dials” for you and present the result on a readout screen.
This assembly is just the voltage divider for the known voltage. It consists of a set of switches and precision resistors arranged so that when you put in a reference voltage, the output equals the voltage you dial in. For accuracy, the resistors used have to be high-precision. These ones have a tolerance of +/- .02% which these days is unheard of. I saw a refurbished working version of this equipment for sale for over $1,000. It’s considered an ultra-precise laboratory-grade device.
From what I can tell this equipment was entirely hand-assembled. That was how it was done in the “old days” of electronics. The colors involved are kind of pretty but they also served to help the assembler be sure he or she had the right part or was sticking the right wire in the right place. All the resistors were made of lengths of fine wire wound onto forms then glued in place with clear paint. Fluke may have constructed the rectangular ones themselves. The yellow cylindrical ones were made by an outside firm.
I couldn’t get over the workmanship put into these components, so I kept one of them. But this one is now extra and is destined for the recycling center.