The '525' designates a washer head machine screw, typically made from alloy steel with a minimum tensile strength of 125,000 PSI. The '10R28' specifies the screw's thread size and length. '10' indicates a #10-32 UNF thread, and 'R28' denotes a length of approximately 1-3/4 inches (or 1-25/32 inches) and a grip length of approximately 1-11/32 inches (or 1-5/16 inches). AN identifies a US Air Force-Navy Aeronautical Standard for hardware. Governed by the AN standard series, administered by Aeronautical Standards Group.