“The mystery of how the inset/recessed license plate is mounted on a early custom car. Well here are some answers. The plate tray itself is a T state, aka tempered type aircraft aluminum meaning it has a spring to it when you bend it. The yellowish green is zinc chromate which can be found on aircraft and at aircraft building facilities. There is also the ever coveted Hollywood Accessory license plate light and some type of galvanized bar mounted to the back and then fastened to studs welded to the body. If you look close you can see tabs on the corners which indicates that it most likely had a piece of glass or lexan aircraft type plastic covering the license plate. A 1947 plate fits right at home. A 1937 plate is too wide.”