Your car was a factory radio delete car.
Having lived the era, many of us who worked at dealerships know the tricks of the trade back then. In many cases, when cars were ordered new, the dealer would write up a U63 radio but order the car without a radio. When the car came in, dealers would install a lesser grade direct fit AM radio to make more money on the sale of the car. This happened a lot. When I worked at Miller Chevrolet, we had boxes of look a like radios in the parts dept that were to be installed in new cars.