No I do not have the manual for it.
I just bought a tascam m-1b line mixer on ebay and it has the same thing on each channel. But I wont have that to play with till next week.
Well, I know on the M1B mixer the foldback outputs are for a couple of different applications.
The first one is to feed the inputs of the optional meter bridge and then the meter bridge has foldback outputs to feed a second mixer for cue mixes as the M1B does not have a built in auxiliary mixing system.
Why your verb has them is a bit of mystery to me too?