There may or may not be a ground wire, that completes the circuit to ground. The early start buttons were different than the later ones, the Y/R wire is grounded when pushing the button, and may just make contact with the housing, which is grounded by clamping it to the handlebar, but not too sure on that one. The later models have the Y/R wire and a G/R wire that go back to the solenoid. They are connected when the button is pushed. I have an old K1 right hand control in the workshop, I will go out and look at it later. I rewired my K1 with a K4 harness, and updated the handlebar switches to the later ones also, with an on/off switch for the headlight.
Charlie