That's an older capacitor checker with a built in signal amplifier for audio use. Before it became cheap to digitally measure capacitors' values, you did it with a device like this. It's basically an electronic bridge circuit that you put an unknown on, and dial around until you get a "balance" indicated with the glass tube sticking out. If I have some time in the next few weeks, I'll do a photo shoot of a similar model in use. There's plenty online if you wanted to look. Capacitor Analyzer or Capacitor Checker with eye tubes.