One thing I thought of just after my reply (after all of this time it JUST occurred to me), is the question of whether there is any correlation to the usage being tied to the same usage period for the rear valance screws. Both of the same screws were used there also but again, during different production time frames (e.g. if a flat screw was used on the buck tag were those same cars getting flat screws as attaching hardware for the rear valance, and the same for the pan head screws?!).