Well went through all my pictures showing unrestored 1970 Metchen built Mustangs & Boss 302's and found none that had black out paint on the inner surfaces behind the grill as we have seen on Dearborn and San Jose cars. Some of the examples below
In additon to these ten cars there are maybe another dozen I have
What I can't address is if the face of the headlight bucket where the headlight was located was blacked out around it. It was difficult to find any pictures of an angle that allowed me to see if this area and the two "wings" mounts on the inner area behind the grill that faces forward.
In the example below of an original paint car appears to show body color in those sections of the headlight buckets that attach to the grill. The same surfaces appear to be visible in body color in the picture above of the blue Mustang