Quick answer
Reference 25866 is a Royal Oak Chronograph. The first generation of that line arrived in 1997 with a 39 mm case, an integrated bracelet and the caliber 2385. Fakes of this reference fail in four places: case thickness, dial print, movement finish and the numbers engraved on the back. A real watch measures about 11 mm thick, has subdial rings that line up with the printed markers, and shows a gold rotor with the AP monogram behind a sapphire back.
Note on sources: Audemars Piguet publishes little for this reference. The figures below come from reference listings for the first-generation Royal Oak Chronograph. Confirm metal, dial and case number before you compare.
What the reference is
Audemars Piguet put a chronograph into the Royal Oak line in 1997. The case measured 39 mm across and about 11 mm thick, with sapphire front and back. The movement was the caliber 2385, an automatic chronograph built on the Frederic Piguet 1185 base. It runs at 21,600 vph and holds about 40 hours of reserve.
The dial layout on this generation: 30-minute counter at 12, running seconds at 9, 12-hour counter at 6, date at 3. Cases came in steel and in gold. Dial colors included blue, black, white and salmon.
Case and thickness
Measure with calipers. A first-generation Royal Oak Chronograph sits at 11 mm thick. Most fakes run 12 mm to 13 mm because the movement inside is a 7750 type with a chronograph module stacked on top. The extra height shows at the lugs and at the caseback ring. Check the bezel screws too: eight screws sit in the octagonal bezel, each slot lined up with the case edge.
Dial checks
- Print. The words Audemars Piguet and Automatic sit on one baseline. Serifs are sharp. Fakes print heavy letters or lose the serifs at the edges.
- Tapisserie grid. Original squares are cut with flat tops that catch light at an angle. Fake grids look shallow or blur into the dial.
- Subdials. Counters sit at 6, 9 and 12. The hands center inside each ring.
- Date. The aperture is cut clean and the number sits centered with a thin font.
- Swiss Made. Two words at the bottom of the dial, split around the 6 o'clock marker on some dials.
Caseback and numbers
Flip the watch over. The back carries a case number and the reference, cut into the metal. Laser printing on top of the surface is a fake sign. Every case number is unique and Audemars Piguet holds a record of it. Fake batches reuse one number across many units, so the same digits show up on listing after listing.
Movement
This reference has a display back. The caliber 2385 shows Geneva stripes on the bridges, hand-polished bevels, and a gold rotor engraved with the AP monogram. Screws sit flush. A fake shows a plain rotor, coarse stripes and machine-cut edges that throw light in uneven bands.
Bracelet, pushers and crystal
The bracelet tapers from the case toward the clasp. Link tops are brushed, link sides are polished, and the joints move without rattle. The clasp folds shut with one firm press and carries the AP monogram. Fakes leave a gap where the end links meet the case.
Pushers are round and screw down. The crown screws down and sits inside the caseband. Sapphire on the original takes anti-reflective coating on the inner face only, so the dial reads clear while the outer surface still reflects. Coating on the outside of the crystal points to a fake.
Paperwork and price
Ask for the case number, the movement number and service records. Audemars Piguet can issue an Extract from the Archives that lists the reference, the dial and the delivery date. Sale prices for 25866 examples move with metal, dial color and condition, so check auction results from the same month rather than trusting a listing figure.