It's a Zinc screw and you can find info in your 67 AIM under the U63 (radio) section. Screw is a 9420152, a self tapping, 1/4-20 x 1/2", with indented head. Can be purchased @Corvette Central #192117 (same head marking as what our 68 had) and Paragon offers "RBW" screws #11189K, 7 screws for $5.00 67/68 Corvettes used these scews for radio ground straps and brake line clips.