I know they never came with a jam nut BUT...
Chevrolet discontinued the Chrome Ball very early as I tried to get one in the early '80's and they were already discontinued by then. The last iteration they sold was a crappy black plastic version smaller than the original and they looked like they belonged on a kids toy. The repro looks pretty good but the threads are no way as good as GM. You need the lock nut just to keep it screwed on.