Short answer: a genuine Royal Oak Perpetual Calendar ref. 25659 is a very thin, hand-finished watch from the late 1980s, and the copies sold as "perpetual" almost never carry a real perpetual calendar. Four checks do most of the work: the movement, the case thickness, the dial register layout, and the stamping on the back. If a seller cannot show all four in clear photos, walk away.
What the 25659 Is
The 25659 sits in the vintage Royal Oak perpetual calendar family that Audemars Piguet built around the ultra-thin caliber 2120, with a calendar module stacked on top of it (the combination often shows up stamped as 2120/2802). Day, date, month, moon phase and a leap year indication packed into a case in the 36mm range. The whole design rests on how little room all of that takes. Reference numbers in this family get mixed up in listings all the time, so a seller who cannot show the caseback stamp and the movement is not really showing you a watch.
Movement First
The base caliber is 2.45mm thick, runs at 19,800 vibrations per hour, and carries a gold rotor. On a genuine watch you should see clean brushing, polished screw heads, and a module plate that sits flush on the base. What you find on copies is usually a Miyota, a Seagull, or a modified ETA under a spacer, or a solid caseback that exists to hide exactly that. Check the winding feel. A 2120 winds smooth and light. A copy feels gritty or stiff. Under a loupe, the engraving on the bridges of a real AP has depth and a consistent font; laser etching looks flat.
Case Thickness and Profile
Ask the seller for caliper photos. Vintage Royal Oak perpetual calendars from this period are among the thinnest complicated watches of their day. Copies need extra room for a stacked module, so they run several millimeters thicker. Feel matters too. The caseband bevels on a real one catch light in a hard, straight line, and the polished chamfer between the brushed top and the side is crisp. On copies that chamfer is soft or wavy. The lugs on the real watch taper and finish by hand; on copies you can often spot the polishing wheel marks.
Dial Register Geometry
Dial variants exist across this family, which is why so many fakes get the details wrong. The subdial centers have to line up with the actual wheel positions inside the module. On a copy the registers look shifted, the printing is a touch too heavy, the moon phase disc sits recessed when it should be flush, and the calendar hands are too wide. Shoot the dial in macro and compare it against auction photos of the same reference. The applied logo, the index feet, and the text weight are where copies fall apart first.
Caseback Numbers and Hallmarks
Vintage stamping is crisp but shallow. Copies tend to stamp deep and even, or use laser engraving that comes out frosted. Check the reference number, the case number, the gold hallmarks, and where "Swiss Made" sits. Then send those numbers to Audemars Piguet for an Extract from the Archives, which confirms the reference, the case number, and the original sale date. A genuine watch with an extract is a different proposition from one sold with only a box and a printed card.
Bracelet and Finishing
The Royal Oak bracelet is a small engineering project on its own. Links should move with even tension, the brushed surfaces should read as a single grain, and the polished bevels should stay sharp along the whole taper. Copies give themselves away at the clasp (shallow stamping, wrong screws), at the end links (gaps at the lugs), and at the edges, which look over-polished and rounded. Weigh it. Gold has a density that a steel copy does not have.
Why "Perpetual" Copies Fall Short
A true perpetual calendar keeps track of February 29 without help. On the 2120/2802 base, the leap year cam and its gearing sit inside the module, and the leap indicator moves. On copies the leap year subdial is often decorative, frozen, or absent. Ask for a short video showing the date advancing overnight and the leap indicator switching. A seller who cannot produce that video is telling you something.
Papers, Provenance and Service
Boxes and certificates are the easiest part of a watch to fake in color. The hard part is history. Auction catalogues list these references with photos, case numbers, and dial variants, which gives you comparables to work from. A service invoice from an independent watchmaker who has handled vintage AP is worth more than a laminated card. If you are spending real money, budget for a watchmaker's inspection before the deal closes. Opening a caseback on a vintage watch with the wrong tools damages the value, so let someone insured do it.
The Legal Side in the US
Trafficking in counterfeit goods is a federal crime under 18 U.S.C. 2320, and US Customs seizes shipments. Beyond that, most competent watchmakers will not service a copy, and module parts do not exist outside the genuine supply chain. A watch you cannot service or resell is not a bargain at any discount.
Fast Checklist
- Movement visible and consistent with a thin 2120 base plus module, not a spacer and a solid back.
- Calipers showing the true thin case, not a rebated copy.
- Subdials centered on the movement, correct print weight, correct hands.
- Caseback stamping crisp and shallow with the right font, and numbers traceable through AP.
- Bracelet edges sharp, clasp stamping deep and consistent.
- A video of the date change and the leap indicator.
- Extract from the Archives or a documented service record.
The short version: the calendar mechanism is the hardest thing to copy, and it is also the easiest thing to test. Watch it run, measure the case, read the stamping, and you will know within ten minutes what is on the table.