There is no known transition date from non crush zone to crush zone cars. While it's the "norm" to see a 71 car with the crush zone style hood, especially later built examples, I've seen a fair number of 71 cars made throughout the 71 model year that originally had the non crush zone hoods. Since the Shaker Hoods were a fairly rare option I believe that they used whatever they had on hand, whatever was in inventory at the plants the cars were built at went on the cars regardless of the style they "should" have had.