Short answer
Audemars Piguet does not list a reference 16402 in its published catalog. No current or vintage AP model carries that reference. The number shows up in replica listings and in used-market posts. A watch stamped 16402 with an AP logo is not a known factory build. It is either a counterfeit or a case number that a seller misread as a reference.
Why the number looks plausible
Older AP watches carry two stamps on the caseback. One is the reference. One is the case number. The case number can be four or five digits, and it sits next to the reference. A buyer who copies the wrong line reports a reference that does not exist.
Modern AP references follow a set format. Digits come first, then letters for the metal. Examples include 15400ST, 15202ST and 26331ST. The letters matter. A bare five-digit number with no metal code does not match AP's system.
Checks on the case
- Reference format. Compare the full string against AP's catalog. A lone 16402 fails this step.
- Serial engraving. Factory numbers are cut into the metal. The cut has depth and clean walls. Printed or acid-etched numbers look grainy and shallow.
- Bezel screws. The Royal Oak bezel holds eight hexagonal screws. On most older models the slots line up along the vertical axis. Slots at random angles point to aftermarket work.
- Caseback fit. The octagonal back seats flush. A gap or a burr near the gasket shows a mismatched or recut case.
Checks on the dial
Royal Oak dials use a tapisserie grid. The squares repeat with even spacing across the full dial. Replica dials show drift near the date window and thin print on the logo. The date font on a genuine dial is small and sits low in the aperture.
Checks on the movement
The AP caliber 2121 beats at 19,800 vibrations per hour. The caliber 3120 beats at 21,600. A listing for a 16402 with a 28,800 beat rate uses a movement AP did not put in a Royal Oak of that period. In-house calibers show Geneva stripes, beveled bridges and a signed rotor. A blank rotor or an ETA base shows a fake.
Papers and price
AP issues an Extract from the Archives for vintage watches. The document states the reference, case number, movement number and sale date. No extract matches 16402. Steel Royal Oak models from the 1990s trade in the high four to low five figures USD at auction. A low asking price confirms the problem.
What to do next
Ask for a full-resolution caseback photo. Compare every digit against AP's list. Send the serial and case numbers to AP's heritage department. If both numbers stay unknown, the watch is not a factory AP.