It's either an internal gain level switch or a voicing switch.....judging by it's positino, I suspect the latter but can't see if it's soldered in/near c5/c14 to confirm it.
I tried out the switch, and I'm still not sure what it does. It changes the tone slightly, but it's not a significant change. I think it seems a little smoother sounding on one setting, but it's very subtle.