If you have PHS then it should still list radio codes, and if not then the option UN9 was the accomodation package too. The electric aerial may have been a GM issue but dealer fitted accessory, think they weren't an option until 79 officially, maybe not. The windscreen aerial was part of the package, if needed or not. Or going off what you said, maybe optioned for good stereo ex-factory but add-on CB/aerial installed later and perhaps not on original order. Buildsheet search starts here!