I read somewhere that when white wall tires were really popular in the 1940's and 50's , tire manufacturers would manufacture all tires as white walls with a thin layer of black rubber over the white wall. When a customer ordered white walls (quite often) the black would be ground off creating a white wall tire. I also remember my dad buying a new Pontiac in 1962 and he thought he was at the top of his game when he ordered the car with Vogue white wall tires (thin white wall).