Hamilton H-31

Hamilton's modified Valjoux 7753 chronograph with 60-hour power reserve. Similar to the H-21 but based on the 7753 (which differs from 7750 in subdial layout: 3, 6, 9 o'clock instead of 12, 6, 9). Features refined kinematic chain, suppressed escapement regulator, improved mainspring, and distinctive mono-block rotor skeletonized with an H-shape and aviation-inspired cut-outs. Used in Pan Europ Auto Chrono and other sportier models.