No expert as always, but if it is correct it's interesting to speculate how Pietro Frua might have planned the whole 428's design from the beginning. Most likely I would have thought he would have been limited to what Italian mass produced major body parts would be or were already available on other Italian luxury cars at the time then drawn the first designs based on them. Which could account for the similarities to the Mistrale and now the rear end treatment of the Fiat Dino coupe?
Given this "existing luxury sports car major body parts already on the Italian market at the time" theory, it might be a way of tracking down other unknown parts' sources as well ? If it's not already known say the bonnet....?