Never saw one of these before.
Here is a link that might be useful: Photo
Is the electrical side polarized? In the picture, it looks like one slot is longer.
When I was a kid, my Grandfather gave me an electronics kit (not a Heathkit, but I had one of those too). It had a radio antenna that had a single spade on the end that just dared you to insert it into a power outlet. I knew better... It was actually meant for this type of shared, in-the-attic, radio antenna.