Short answer
IW376091 follows IWC's reference format: the letters IW plus six digits. The first four digits point to a case and movement family. The last two mark a variant, often dial color, strap, or market. A seller who quotes IW376091 describes a watch by that number. The number does not prove the watch matches it. Check the caseback engraving, measure the case, and ask which caliber sits inside before you pay.
What the number encodes
IWC stamps the reference on the caseback and prints it in the warranty booklet. On some models a second stamp sits between the lugs. The digits match across all three or the paperwork is wrong. Ask for a caseback photo shot at an angle. Laser etching looks shallow and gray under raking light. Cut engraving has visible walls and a darker floor.
Checks on the watch
Case and crystal
Measure the case with calipers at 2, 4, 8 and 10 o'clock. Four readings within 0.1 mm of the stated diameter is normal. Two readings that differ by 0.4 mm point to a case cut from a generic blank. IWC fits sapphire with anti-reflective coating on both faces across most lines. Under a lamp the coating shows a faint blue tint on each side. A single-sided coating shows tint on one face.
Movement
Ask for the caliber number and a photo of the rotor. Replica chronographs in the $250 to $600 band often carry a Sellita SW500, an ETA 7750 clone, or a Chinese 7750 copy. Those run at 28,800 vph and the chronograph hand jumps on start. Genuine IWC chronographs use IWC-modified or in-house calibers. Get the caliber number in writing and cross-check it against IWC's published data.
Water resistance
ISO 22810 sets the test requirements for watch water resistance. A seller who claims 300 m with no pressure-test record is guessing. Ask for the test result and the test date. Check that the crown and caseback threads engage within one turn. Loose threads on a replica case leak at 3 bar.
Dial and hands
Look at the base of each applied marker under a loupe. Genuine markers sit flush with no glue halo. Replica markers show a shadow line at the foot. Lume tone should be even across the dial. Bright white lume on a ten-year-old watch means a relume or a mismatch.
Price signals
Replica listings for this reference run from about $250 to $900. Under $200 often means a 21,600 vph movement, a mineral crystal, and stamped markers. Above $900 adds no finish. At that price the seller is charging for a box and papers that do not exist for a replica.
Legal note
In the US, trafficking in counterfeit goods is a federal offense under 18 U.S.C. 2320. A replica sold as a replica is a separate matter from a replica sold as a genuine IWC. Keep the listing language clear on both sides.
Questions to send the seller
- Case diameter measured with calipers at four positions.
- Caliber number and beat rate.
- Pressure-test result and test date.
- Caseback engraving photo at a 30 degree angle.
- Return window and who pays return shipping.