Stereo Input on R75

[mrtidy]

In my pursuit of all things Rickenbacker, I purchased a 1979 R75.
The two individuals channels by themselves are nothing special, but if I plug my mono guitar into the first "click" of the Stereo input, both channels seem to work at the same time and it sounds great with a much heavier bottom. If I plug the cord all the way in, both channels still work but it's not quite the same.
I would have guessed that each pickup on a stereo guitar would have it's own channel with this input. I don't have a stereo cable to test this out.
Did that make any sense?
Maybe someone with more electronics experience can look at the schematic (available on the Rickenbacker website) and clue me into what the stereo input is supposed to do and if mine is functioning correctly.
by the way, mine is signed on the inside by "Mary #2" and "Vicki"