The first '3' indicates a bolt diameter of 3/16 inch. The second '3' indicates a grip length of 0.063 inches and a total length of 0.469 inches. The 'H' indicates that the head of the bolt is drilled for safety wire. The 'A' indicates that the shank (threaded end) is undrilled, meaning it does not have a hole for a cotter pin. Army-Navy standard, identifying it as US aerospace/military hardware. Governed by the AN standard series, administered by Aeronautical Standards Group.