antenna on vette

where can i find th epart the screws down my antenna to the body of my car.  There is a threaded portion on trunk end of the car and I have the antenna but it looks like something that would slide down the antenna and screw onto the car holding the antenna in place. I had it at one point but i cant find it. i needed to take it off when I covered my car at the in-laws in their garage while my house was being worked on.

jeremy