The short answer
Audemars Piguet stopped making the Royal Oak ref 5402ST in the mid-1980s. No authentic 5402 is produced today. Any watch sold as a new or boxed 5402 is a counterfeit, a franken build with mixed genuine and aftermarket parts, or a re-cased watch. A caliper and a movement photo settle most cases before money changes hands.
What the reference 5402 is
Gérald Genta designed the Royal Oak. It debuted in 1972. The 5402ST measured 39 mm across and about 8 mm thick. It used the calibre 2121, an ultra-thin automatic that Audemars Piguet, Patek Philippe and Vacheron Constantin each derived from the Jaeger-LeCoultre calibre 920. The dial is a Petite Tapisserie grid. The date sits at 3 o'clock. The caseback is solid steel with cut engraving. Series letters run A, B, C and D. Dial print, logo placement and bracelet details change between those series, so a comparison watch must match the right letter.
Where counterfeits fail
Thickness
The genuine case is thin. Most replicas measure 10 mm to 12 mm because the case holds a modified ETA or a Chinese automatic. Ask for a caliper photo of the case back to crystal.
Movement and caseback
A 5402 has a solid caseback. A display back means the case is not a 5402 case. Genuine engraving is deep and cut. Many fakes laser-etch text that sits shallow and flat.
Bezel screws
The eight hexagonal bezel screws have a slot cut into a hex head. Cheap copies use flat screws with a hex shape printed on top, or screws that do not align with the caseback bolts.
Dial
Petite Tapisserie squares are raised and spaced even. Fakes show soft edges, uneven gaps and date windows placed too far from the movement ring. Check the date font and the printing of the brand name.
Bracelet
The integrated bracelet tapers from 20 mm at the lugs. Link edges are chamfered. Replica bracelets rattle and end links sit proud of the case.
Checks before you buy
- Ask for a caliper reading of case width and thickness.
- Request photos of the movement and the caseback engraving.
- Send the serial and reference to Audemars Piguet for an Extract from the Archives.
- Compare auction results for the same series letter.
- Treat a steel 5402 priced under $10,000 as suspect. Auction results for clean examples sit far higher.
Limits of photo checks
Photos hide thickness and bracelet feel. A franken watch can use a genuine case, a service dial and a swapped bracelet. An in-hand exam by a watchmaker with AP parts knowledge is the only test that closes the question. Auction houses, Audemars Piguet and a small number of independent specialists do this work.