The 16" were a heavy duty option. It has nothing to do with age. The 2 or 3 piece which are commonly called semi drop center are for heavier ply tires. The one piece were for light duty tires until tubeless tires came out. Tubeless tires require a drop center wheel. Semi drop center are being phased out, but they are still used on container chassis that sit around on the bad side of town because no one bothers to steal them. There is not the selection of tires available for tube required as there is tubeless.