antenna hole plug

The '75 Duster I have as a donor car came with a radio delete on it and the factory plug looks like the standard chrome antenna bezel on the outside then there is a round black rubber or plastic ring then on the inside diameter there is a small chrome circle with a chrome 5/16" bolt head. I'm assuming it's a plug that when bolted down expands tight to fit. I'll see if I can take it off this weekend and see what the set up is. It looks pretty clean.