Interesting - if you look at the "non-illustrated parts" sheet for Z28 in the 1968 assembly manual, it shows that the fan and drive assembles the same as K02. If you look in the K02 section (sheet A1), it shows the "production reinforcement" between the water pump hub and the back side of the water pump pulley. Then if you go to UPC 6, sheet A7, which shows the "production" V-8 pulleys, there's no mention of any reinforcement, and the nomenclature shows just "pulley", not "pulley asm", which would be the case if the pulley had a reinforcement welded to it.
Corvettes of the era had the reinforcement spot-welded to the back side of the pulley, and there was a TSB to add it to cars that didn't have it.