This one is a switchbox for loudspeakers (up to 2 pairs). There's a switch to reverse the polarity of one side (in case you wired it wrong, or aren't sure and want to compare) and volume controls (this was by the bed where I could reach it; the amplifier was across the room) and headphone jacks.
...I actually don't remember how all the switching worked; I just remember that it was definitely for *speaker*-level signals, not line-level.
Also, I used bundled phone-wire cables (soldered to DIN-5 plugs) to carry the signals.
Crosspost: 2019-11-20 TootCat