There is still a possibility its factory. At least here in Sweden we had a car phone system in the 69-70s, very expansive, very few subscribers. I’ve had a Volvo 67 with a phone installed. Not sure it was factory though.I’m looking at a 79’ Fleetwood and the owner told me the phone is factory and very rare. The trunk deck has two antennas drilled in. Has anyone heard of this before?