[Tynwald]

Yes, the third wire goes to the stud on the side and is separate from the other two.

I've usually used cloth covered cords and era specific plugs but this one was so funky that I was tempted to use it somehow. Guess I'll just put it aside as a curiosity and just do what I've generally done with power cords and use a completely separate antenna wire. That wire should terminate at some sort of connector so one doesn't have to get access to the chassis to connect the antenna, which is more difficult to in chairsides than in most consoles. That would more-or-less serve the same purpose as the antenna stud on the old plug. 
Anyway, thanks to everyone.