I have seen it ONCE in all of my life. It cost me a fifty mile service call on a tractor we had just sold a fellow. It was a JD 3020 gas. He had boughten it from me a few months before. He had wanted the parts needed for a tune up included in the deal. He had ran it for a while and then changed all the oils and filters. HE did a tune up on it at the same time. HE called me as the tractor has started and ran maybe ten minutes an then died. He could not figure it out. It took me a while to find it too. I still have that rotor button in my tool box. It is like yours in that it has zero continuity between the center contact and the end.
So yours makes the second one I have every heard of. The one I had was a Delco system.