This Gammill 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 on/off switch on non-stitch regulated machines on the right-hand side.
It can also be used for the left handle switch on various Gammill models. It has a white top that screws on/in.
(Some non-stitch regulated models use the 3-prong switch on both the right and left handle, where others use a 3-prong and a single stitch.)