Guess that if the hole has been used to add the L bracket by a PO then that would remove that car from the sampling but we should be able to tell a punched hole from a screw hole
Will see what I have in pictures when I find some time
An added screw hole would be obvious, there is also a raised crease below the hole for strength.
Today at the Hillsboro all Ford Show, I saw another late 1969 Cougar with the apron, dated 08G. Several 1969 Mustangs in attendance, but the two with the 1970 style battery aprons, were much earlier cars, and had obviously been replaced.