My club has several of these "relics" in use - perfectly usable, BTW.
The left knob is the volume control and the right is a "zero adjust".
The switch is a 3 position on/off, audio up only, and audio up and
down. Before take off, I would set the audio to "up only" and set the
"zero adjust" at the point where the audio was on the edge of on/off.

Tony V "6N"