Keep in mind service replacement were made many years after the same parts were no longer being installed at the assembly plant. Manufacturing continues on worn out tooling, suppliers change, which can create variations. With the '66 grille, you're seeing one of those service variations. There are original cars that have a light coating of paint on the back, but usually it's at least a light to medium coating, with shadows in certain areas.
Another detail to look out for on those grilles is the rivet style. There were some produced with open rivets, like a pop rivet. Originals should be closed/domed on the top.