Short answer
"11 45100" is not one Audemars Piguet code. It is almost always two separate stamps lifted from the same caseback photograph: a short two-digit case or caliber stamp, and a longer five-digit serial or movement number. On a genuine watch these figures agree with each other, with the movement inside, and with the guarantee card. When a listing shows them as a single string, or when the engraving looks airbrushed, laser-dotted, or oddly spaced, treat the watch as suspect until a service center confirms it.
Where the two numbers usually come from
An Audemars Piguet caseback carries several distinct groups of text: the brand name, the full reference number, a serial number, and a caliber or movement number. Sellers often shorten this into listing titles, which is why buyers end up searching for fragments like 11 and 45100. A short stamp such as "11" can be a case batch mark, a caliber prefix, or the leading digits of a longer serial. A five-digit figure such as 45100 reads as a serial or a movement number. Neither number identifies a watch on its own, and neither can be checked from a photo caption.
How genuine AP codes are laid out
Reference number
A modern reference runs in blocks separated by dots, for example 15400ST.OO.1220ST.01. The opening digits identify the model and collection. The letters that follow describe the case material, with ST used for stainless steel. The next block covers the bracelet or strap, and the final digits show the dial or buckle variant. A watch listed as steel should never carry a gold material code on the back.
Serial number
The serial sits on the caseback and repeats on the movement and on the guarantee card. AP serials are not sequential by year, so you cannot date a watch from the serial alone. What you can do is compare all three locations and look for any difference.
Caliber or movement number
Calibers such as 2121, 3120, 4302, and 4401 appear engraved on the movement bridges and rotor, not on the outside caseback. If a seller quotes a movement number that never appears inside the watch, that is a fault, not a typo.
Prerequisites
- Macro photos or a 10x loupe
- Clear images of the caseback, the movement through the display back, and the guarantee card
- The full reference number, including both dots and all letters
- Access to an authorized AP service point or boutique
Checking the codes step by step
- Photograph the caseback in raking light so the engraving throws a shadow.
- Write the full reference number down, including every dot, letter, and digit.
- Compare the serial on the caseback with the serial on the guarantee card and the movement.
- Match the material letters to the actual metal of the case and bracelet.
- Examine the engraving depth and edges: genuine stamps cut sharp, even grooves with no laser dot fill.
- Count the digits. A five-digit serial where a reference uses six is a mismatch worth flagging.
- Check the caliber number against the movement visible through the caseback.
- Look for glued-on or drilled-out numbers, which show as a bright halo around the letters.
- Compare the watch side by side with a known genuine example of the same reference.
- Request an archive extract or authentication from Audemars Piguet before you commit.
Red flags in engraved codes
- Font weight that changes between letters on the same line
- A serial that also appears in another public listing
- Reference digits that do not match the case shape or dial layout
- Misspelled material codes or a missing variant suffix
- Numbers that sit crooked to the caseback center line
Why matching codes still do not prove authenticity
Counterfeiters copy numbers straight from genuine auction and forum photos, so a correct serial means little by itself. Two watches can carry the same stamp. Only Audemars Piguet can confirm that a serial left the factory on that exact watch. Be wary of any website that claims to decode an AP serial in seconds, because AP does not run a public online serial lookup.
Dealing with a listing that says 11 45100
Ask the seller for one unedited photo of the whole caseback and for the full reference number. If the two figures cannot be split into a reference, a serial, and a caliber, or if the seller refuses close-ups of the engraving, stop there. A watch that survives that first check still deserves a hands-on inspection before money changes hands.