Old mixer with spring reverb from Grandpa (problems)

[hambloksem]

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I have a couple of questions about my grandpas old mixer I got from him.
It's the brown box between the pioneer amplifier and behringer eurorack.
Does anyone know what I can do with this switch insert? Is it for a pedal of some sort?
This is an old mixer from my grandfather which can mix 8 tracks and has a spring reverb in it.
Also for some reason I can't mix sounds stereo and the sound will only come out of the right side of the phones(connected to the out)I fixed this by using a jack splitter so I have 2 different out channels you can see this in the last picture. Anyone knows why I don't have stereo? Do you think it is broken? When i plug something in the bottom row of channels like the red cabel black cable it just comes out of the right speaker like the top row.
I really like the spring reverb only sad that There is no stereo out...
Anyone ideas?
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Thanks in regard,

[BertKoor]

It's not an 8-channel but a 4-channel mixer, all in glorious MONO like most affordable gear of that era.

Each channel has two inputs: "H" and "L" with the label "IMPED" next to it which stands for Impedance. Guitars and certain types of mikes go into Hi, line level and other types of mikes should go into Low. Yes, you can duplicate both channels and listen to dual-mono on your headphones, but in order to make it true stereo you need to duplicate the whole internal board, fit stereo pots, etc etc. The spring reverb unit also is mono. To get that in stereo you need two units, not one.

The "switch" jack is probably to switch the reverb on & off with a simple foot pedal that makes/breaks contact from pin to sleeve of the jack.

So it's not broken but just works according to the specs (in mono)