This is a Trilobe, thread forming, or thread rolling screw, ideal for applications where the screw is being threaded into plastic or metal.
A flat washer is part of the 5/16 hex head, providing more surface area contact than a regular hex head. The increased bearing area decreases the likeliness of crushing mating surfaces. Trilobe, or Thread Rolling, screws have a blunt tapered point that requires a pre-drilled or punched hole; eliminates chips and permits replacement with standard screw. Zinc plating froms a zinc carbonate film that greatly retards the corrosion rate in a normal atmosphere, which is similar to the aluminum oxide film that forms on aluminum that accounts for its low corrosion rate.
'Work great, but you will need a socket and good size ratchet to get them in. They are very tight and also get very hot when you start ratcheting them down (enough to burn you).'