I have a couple of these. Of the ones in a working device, they are in capacitor ("condenser") checkers. You rotate a dial until the eye tube goes from closed to open, then read off the capacitance on the dial.

I don't remember the exact design off hand, but I seem to remember that about all that's inside one of these is the eye tube, a rectifier tube, and a handful of resistors and capacitors. Can also test leakage and shorts. They really knew how to get a bunch of things done with about 10 components back then.