50FNSJ5 1/2-20 Slotted Jam Nut (Castle Nut) is used in the plate where a cotter pin is needed to secure the nut from backing out. 

Castle Nuts are commonly used through the kart or racing chassis as part of safety tech. They are secured with either cotter pins, r-clips, or safety wire in cases where a critical component like a header or wheel hub could detach from the engine or chassis and injuring a driver or spectator. 

This particular castle nut is used on the spindle kingpin for racing go-karts, racing lawnmowers, and other components with 1/2-20 thread.