GAS Thoebe preamp: Stereo push buttons?

I just picked up this preamp.
It has 2 stereo push buttons marked stereo A and stereo B

The preamp refers to the left and right channel as A and B.

I am at a loss as to what these buttons do. When both are
pressed it sounds like the system is in mono mode.

anyone out there have a manual ?

thanks
craig in ottawa.