GAMMILL ON/OFF HANDLE SWITCH part #81-1902B
This switch has three wire terminals at the bottom. The switch toggles, that is, it closes the circuit between center (common) and the right leg, then when you click the switch it breaks that circuit and closes the circuit for the other leg.
It is used for the needle positioner and the left handle switch on various Gammill models. It has a white top where the button screws on/in.