These are nothing more than blocks of wood that are rabbeted to form a tongue on one end. This tongue then fits into a kerf in the rails.
To make the fasteners, I start by planing down a long, 3⁄4"-wide blank. Using a straight bit at the router table or dado blad at the table saw, cut a rabbet on the ends of the strip to create a 1⁄8" tongue. After drilling a pilot hole for the screw, you can cut the fastener to length.