The Royal Oak Perpetual Calendar joined two of Audemars Piguet's defining traditions: ultra-thin calendar watchmaking and Gérald Genta's steel sports design. Reference 5554, later renumbered 25554, entered serial production in 1984. Its four-register display tracked day, date, month, moon phase and leap-year cycle inside a case that remained remarkably slim. At a fragile moment for mechanical watchmaking, the model helped demonstrate that a high complication could live in an informal steel watch.
Reference 25654 became the best-known vintage generation, followed by the 25820 and related openworked versions. Small production differences—dial signature, leap-year indication, moon disc, typography and bracelet—carry real importance. These watches rely on the thin 2120/2800-series base, so calendar adjustment must be done with care and never through forbidden hours when the mechanism is engaged.
The 41 mm 26574 modernised the family with caliber 5134 and an outer week-number scale. Ceramic references, especially the black 26579CE, showed that the integrated bracelet could be executed in technically demanding materials without losing its finishing pattern. The current 26674 generation introduces caliber 7138 and crown-controlled correction, allowing the calendar indications to be set without separate case pushers. Whether vintage or current, verify that every display advances in sequence, the moon phase and leap-year cycle are correctly synchronised, and the watch has documented specialist service. A perpetual calendar that appears cosmetically perfect can still require a costly calendar overhaul.