Rick, the guy I spoke to about these plugs was out in California when I was calling around looking for the original style plug for my car. He was the one who gave me the info and he seemed to be very knowledgeable. I can't remember the name of the place but they dealt only with old car/musclecar electrical items.
He did state that the grey plugs were used on Big Block cars but like I said, earlier. I have never heard of a black colored plug going to this switch in the head but who knows there could have been a couple different manufacturers making these things back then for GM.. Plus it obviously had to be there if you observed it on some original, survivor cars...
Rich