The mystery switch is indeed wired to the altitude correction plug. The rocker switch has three wires, black, grey, and white from top to bottom that go into a sleeve. Sometime in the past the black and grew wires were connected to the male portion of the altitude plug, which looks like it has two black wires. The free end of the white wire remains in the sleeve. It looks like I've answered my question on which position is "on", which would be up. While I'm not happy with the connectors used, I think I'll keep it active in case I ride to elevation over 4000' or so. The discussion on which pins are connected is interesting. My understanding of the altitude correction plug is that when inserted it interacts with a pin otherwise not used, changing the amount of fuel. I would assume it would lower the fuel with higher altitude, just as changing to a smaller jet in a carb would do.