Factory D/Dart 1966. Supposedly only 50 were made. The water gets muddied a bit because Mopar put out a bulletin that detailed a retro-fit for 274 4 bbl cars.
My second choice would be any sort of factory installed stereo radio. Would not include the cassette unit as it was not integral to the radio.
The 1966 Dart Owner's manual listed AM/FM radio, rear speaker, and reverb unit as dealer installed accessories. Does anyone know if the reverb unit was ever factory installed after 1966? When did Mopar quit offering it? I have only seen one, it was in a neighbor's 65 New Yorker.