69 Dart Remote Mirror Question

Hey guys,
I am installing the left side view mirror on my dart. It is remote and the cable comes thru the top of the inner door as you may know. There is a ring that fits on the remote handle and I think it goes behind the upper inner door skin. My question is: this is a repo mirror and the ring I spoke of has 3 tabs on it. Once it is behind the door, do I spread the tabs open against the inner door? I couldn't find any reference to this ring in my parts or service manual. The ring has 2 tabs the same size and the third is smaller. The door has a small slot cut in the inner door skin that the small tab will fit in and I don't know why. Here are some pics. Thanks,