I've always understood the "stud" (prong about 3/4" long ?) was there to ensure that the wheel could only be refitted in one position as it had to coincide with a single hole in the wheel.
It used to be possible to get wheels balanced on the car, the balance being lost if the wheel was ever replaced in a different position on the wheel studs. For some reason that doesn't seem to matter now - perhaps hubs and disks are better balanced in manufacture?
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