This lock disc sandwiches between the two hub nuts and has a tab that slides down a groove cut along the length of the spindle. One of the hub nuts on each side has a pin. It should go on first with the pin pointing outward. Keep adjusting the nut so that the pin fits into one of the holes on the lock disc once it is slid into place. Now thread on the second hub nut and tighten it down with the proper tool. Finish it off by installing the locking hubs.