Fender/Body Speed/U Nut 5/16-18 - Black Oxide
Extruded Steel UNut, 5/16-18 in thread with Black Oxide finish OE style.
Ford/Mopar style, sometimes also known as J Nuts or Speed Nuts, commonly used to attach fenders and other main body panels.
Auveco has been manufacturing and distributing specialty automotive and industrial fasteners for over 100 years. They are the world leader in Automotive Body Hardware and Specialty Fasteners.