The Cam Key serves two purposes. It secures hone half of the Wave Cam and it holds the kingpin locknut in place. If your trucks "click" or "wobble" you might need a new set of these. That's rare though. This is a highly unusual part to need to replace. But if yours is worn out or rounded over, it's worth a try.
Sold in sets, just enough for both of your HST Trucks.