That is a two-speed switch. When you twist the shaft on the switch, you should get two clicks. The plug is for a variable speed switch (which has a prong near the center where that switch doesn't). If you can find a variable speed switch and plug in the two connectors, you'll still have one prong empty on the back of the switch, but it should work fine. I think the extra spade is for a ground, but the switch grounds on the mounting frame.