Maybe the plain black wheels were the more ecomonic (cheaper) version. Wasn't the Bar-B-kettle a opening price point option, then you moved up in price and features from there. I understand that the kettles had already came with the white walls, but maybe Weber replaced the white wall with the plain black along the way. Maybe to keep the cost of the grill down. But then for the higher price point grills they kept the white walls. I know I have some catalogs from the 80's and 90's that I can check on this.
Oh, and very nice shine on that classic kettle.