Reference 16402 does not exist in the Audemars Piguet Royal Oak catalogue. The number is likely a mashup of 16202 and 15400, and any listing, invoice or forum post that calls a watch a "Royal Oak 16402" is working from invented paperwork. That is the fastest filter you can apply, because the number is a documentation problem before it is a watch problem. The checks below follow the order a watchmaker would use: reference plate first, then dial, movement view, bracelet and seller conduct. No single check proves anything on its own, so weigh them together.

Why 16402 Falls Apart On Sight

Audemars Piguet reference numbers for the Royal Oak are well documented. The Jumbo line runs 5402ST (1972), 15202 and 16202. The 41 mm line runs 15400 and 15500. Mid-size pieces sit around 14790 and 26300. Nothing lands on 16402. Sellers who use it usually do one of three things:

• Copy the wrong number from a stock photo of a different reference.
• Blend two real numbers to sound more official than the watch is.
• Reuse a supplier's catalogue code and pass it off as an AP reference.

If a seller cannot state the correct reference and back it up with the caseback, assume the rest of the description is loose too.

Check One: Reference Plate, Caseback and Engraving

Ask for a macro photo of the caseback and the reference plate between the lugs. Look at the font weight, the depth of the engraving, the serial placement and the spacing between characters. Laser engraving tends to look shallow and even, while stamped engraving has a slight raised edge you can catch with light from the side. Spelling errors, wrong case numbers and serials that repeat across listings are common.

• Pros: Fast, needs only photos, and catches careless listings immediately.
• Cons: Engraving can be copied closely, photos can be retouched, and a genuine caseback photo tells you nothing about the watch attached to it.

Check Two: Dial Texture and Date Window

The Jumbo dials use Petite Tapisserie. The 41 mm Royal Oaks use Grande Tapisserie. The squares should be evenly spaced, cut cleanly, and aligned so the pattern meets the rehaut without drifting. On the Jumbo, the date window sits without a metal frame and the aperture is cut square. The applied AP logo should sit flat with crisp edges, and the printed name should be sharp at high magnification rather than soft or doubled.

• Pros: The dial is the most expensive part to copy well, so this check exposes cheap builds quickly.
• Cons: High tier builds get dials close enough that low resolution photos hide the difference. Ask for daylight shots at an angle, not studio flash.

Check Three: Thickness, Profile and Movement View

The Jumbo is 8.1 mm thick because the movement is ultra thin. That is hard to match, so a caliper reading on the crystal and caseback is one of the most useful numbers you can get. Anything reading well over 8.5 mm on a watch sold as a Jumbo should raise questions. With the caseback off or through a display back, look at the movement architecture, rotor engraving depth, jewel count, and whether the bridges look machined or simply decorated plates over a base calibre.

• Pros: Thickness and movement construction are expensive to fake and hard to fake convincingly.
• Cons: You need the watch in hand with calipers, and some sellers will not allow measurement before payment.

Check Four: Bracelet, Clasp and Screws

The Royal Oak bracelet tapers toward the clasp and each link edge is chamfered and polished against a brushed top surface. Look at the hex screws: the slot should be clean, centered and consistent across all eight, not chewed or off centre. The clasp stamping should have depth, and end links should fit the case without gaps or rattle.

• Pros: Bracelet finishing is a reliable tell and easy to photograph.
• Cons: A genuine watch may be wearing an aftermarket strap, and the best replica bracelets are close enough that photos alone will not settle it.

Which Route Fits Your Situation

Buying from a marketplace listing sight unseen: treat the 16402 number as a stop sign, then request caliper photos, a caseback macro and a short video of the seconds hand running. Buying a legitimately numbered Royal Oak: go through a dealer or auction house that provides service history and papers, because no photo checklist replaces provenance. Buying a replica knowingly, for study or wear: choose a seller who shows real photos of the actual unit, states the case dimensions, offers a return window, and never presents the watch as genuine. Inherited or gifted a watch: take it to an independent watchmaker for a caliper measurement and a movement inspection before you spend anything on service.

The honest summary is that reference 16402 is not a model, it is a clue. Start there, move outward through dial, movement and bracelet, and expect the answer to come from a measurement rather than a photograph.