Quick answer
A case back stamped with a two-digit number over a five-digit number, such as 11 over 45100, is not a genuine Audemars Piguet reference. Genuine Royal Oak and Royal Oak Offshore references use a five-digit number in the 1xxxx or 2xxxx range, followed by period-separated material and bracelet codes, for example 26331ST.OO.1220ST.01. A bare two-digit stamp sitting above a five-digit number that begins with 4 does not match that format. Treat the watch as counterfeit unless a full, verifiable reference string proves otherwise.
How Audemars Piguet numbers its watches
Audemars Piguet splits its case back engraving into separate fields. The reference number identifies the model. A material code follows it, using two letters such as ST for stainless steel, OR for rose gold, BA for yellow gold, TI for titanium, and CE for ceramic. A bracelet or strap code comes next, then a two-digit variant. A separate serial number is engraved nearby, and on many models the movement caliber is stamped on the rotor or the movement bridge rather than on the case back.
Two-digit numbers do appear on genuine watches, but as a variant suffix at the end of a full string, not as a standalone prefix floating above a five-digit figure. That detail is where the 11 / 45100 layout fails.
What 11 / 45100 usually is on a counterfeit
Counterfeit engravers often copy a number from a listing photo without understanding the structure. The result is a plausible-looking pair of numbers with the wrong hierarchy, wrong font weight, and no material or bracelet codes. The 45100 block falls outside the standard Royal Oak reference ranges, and the isolated 11 has no defined meaning in AP's catalog. The stamp looks official to a casual buyer and falls apart under a loupe.
Prerequisites
- A 10x loupe or a phone camera with macro mode
- Access to the official Audemars Piguet reference list
- A digital caliper for case diameter
- The seller's warranty card or service paperwork, if available
Steps to verify a watch stamped 11 / 45100
- Photograph the case back in flat, angled light so the engraving casts a shadow and every character is readable.
- Write down the complete reference string exactly as engraved, including any characters before and after the slash.
- Compare the string against the official Audemars Piguet model list and against a published reference database.
- Check whether a two-letter material code appears directly after the five-digit reference.
- Look for a period-separated bracelet code and a final two-digit variant at the end of the string.
- Inspect the engraving itself under magnification and compare stroke depth, edge sharpness, and letter spacing against known genuine photos.
- Measure the case diameter and thickness with the caliper and compare those figures to the specs of the claimed reference.
- Ask the seller for the warranty card, which prints the reference in full, and compare it character by character.
Red flags that confirm the stamp is fake
- A five-digit number starting with 4 that matches no catalog entry
- No material code, no bracelet code, no variant suffix
- Shallow laser etching with rounded edges instead of deep, crisp cuts
- Letter spacing that drifts wider or tighter across the line
- A serial number that does not match the movement or the paperwork
- Case back screws with uneven slot depth or a mismatched finish
If two or more of these signs are present, stop the deal. A genuine AP reference can always be traced to a full catalog string, and a seller who cannot produce one is selling a fake.