I have a set of rusty originals and they have the same black paint on them. I don't think it was a SS blacked out panel thing. Mine came off a Z28. Also Blacked out rear panels didn't extend below the rear bumper that far. Probably just a GM thing to hide the bracket part to not distract from the bumper guard itself.
On a '69 Camaro the last six digits of the VIN and the 'ident number' number were on the body and chassis broadcast copies. The ident number is the number on the body tag. The full VIN and body tag number were on the window sticker & shippers' copy.
The VIN was not stamped on the body tag on cars shipped to the states. Some export Camaros did have the VIN on the tag; others had an additional tag that had the VIN stamped on it.