These plates were originally designed for the 68 1/2 428 CJ Mustangs. So the 428 markings were used for in plant identification for this unique one car, one engine application. As the model years went by, use was expanded to the Boss, other 428 powered Mustangs, and the 71-3 competition suspension-4bl Mustangs. And as David can tell you, as the worn stamping dies are repaired/replaced, the 428 designation was left off since it was no longer a unique one car, one engine part. So you'll see some plates with "428" and others without. Since the Cougar and Mustang shared the same chassis the plates would be the same.