A look back in truck model history may lead to the answer. If they continued using the punch machine setup from an earlier model truck for the AD trucks, changing that setup for the new model which did not require the slots would cost time. Time is money.
A similar deal puzzles me with the front and rear bumpers with the extra, unused center hole. There was even a dummy bolt for plugging the extra center hole. It was held on by a push-on 'dang it' clip.
Carl
