I believe this is what you are describing? It was not part of the radio but is used to adapt a modern Motorola type antenna plug to the radios GM style twist-lock connector. Not a very reliable connection to the antenna socket in my opinion; I prefer to make an adapter from an original GM connector and a Motorola socket and a short piece of antenna cable. Or, install a Motorola socket on the radio in place of the original GM type.
I got the pictured adapter sleeve from Y&Z's yesteryear parts, but they are not listed in their catalog and may not still be available. A phone call to the friendly folks at Y&Z might find out.http://www.ynzyesterdaysparts.com/