Short answer
A fake IW371640 gives itself away in three places: the dial printing, the movement behind the sapphire caseback, and the engraved numbers on the back. Start by confirming that the reference number actually exists in IWC's own records, because replica listings regularly quote reference numbers that do not match any catalogued production variant. If the number checks out, move to the dial, then the case, then the movement, then the paperwork. Any single mismatch is enough to walk away.
Prerequisites
- A 10x loupe or a macro lens for phone photos
- Bright indirect light and a plain white surface
- Sharp photos of the dial, caseback, movement, and strap from the seller
- The reference and serial numbers exactly as printed on the watch
- A way to reach IWC customer service or a boutique for verification
- Calipers if you can measure the case yourself
Inspection steps
- Send the reference and serial numbers to IWC customer service and ask whether the reference exists and which dial and case variants it covers.
- Measure the case across the bezel and across the caseback, then compare the numbers to the published specification for that reference.
- Measure case thickness. A counterfeit case often runs noticeably deeper because the movement inside is thicker than the in-house calibre.
- Study the dial text under magnification. Genuine printing sits on a single plane with even spacing and sharp terminals; counterfeits show fuzzy edges, ink bleed, and drifting letter spacing.
- Check the applied indices and numerals. Real ones are faceted with a polished bevel and a clean brushed top; fakes are flat, softly stamped, or sit at a slight angle to the printed track.
- Compare the two subdials. On the Portugieser Chronograph they sit on a vertical axis at 12 and 6, and the snailed rings should be perfectly concentric with hands that reach the ring edge.
- Press the chronograph pushers. A column-wheel chronograph starts, stops, and resets with a light, distinct action; a converted cam movement feels gritty and the reset snaps hard.
- Run the chronograph for ten minutes and confirm the elapsed minutes hand tracks cleanly without jumping or stalling.
- Trace the case edges at the lugs and bezel. Genuine cases have a crisp line between polished and brushed surfaces; fakes blur the transition or polish areas that should be brushed.
- Turn the watch over and read the caseback engraving. Look at depth, font weight, and spacing. Shallow laser etching and font substitutions are common on counterfeits.
- Photograph the caseback numbers and compare them with the numbers on the warranty card and any service paperwork.
- Inspect the movement through the display back. Confirm the column wheel, the shape of the balance bridge, the calibre engraving, and the finishing on the plates. A rotor stamped with a calibre number that does not correspond to the visible bridges is a clear sign of a counterfeit.
- Check the crown and pushers for correct height, signed logo, smooth winding, and a consistent number of turns before the crown seats.
- Examine the strap or bracelet hardware. Counterfeit straps use thinner leather, uneven stitch density, and generic buckles with soft stamping.
- Compare the asking price with current used-market listings for the same reference in comparable condition.
Why the movement decides the case
Dial flaws can be argued about. Movement architecture cannot. IWC's Portugieser Chronograph uses an in-house column-wheel chronograph calibre, and no budget counterfeit reproduces that architecture faithfully. Fakes usually hide a modular add-on chronograph or a decorated third-party movement under a rotor engraved to look correct. Ask for a clear, well-lit photo of the movement with the chronograph engaged. If the seller refuses, treat that as an answer.
Paperwork and price signals
A genuine watch in this reference range comes with a warranty card whose serial matches the caseback, and often with service records from an authorized center. Registration for the extended warranty program requires a serial that traces to a genuine watch sold through authorized channels, so a seller who cannot complete registration is telling you something. Price matters too. A steel Portugieser Chronograph in good used condition trades in the thousands of dollars, not the hundreds. Listings at a fraction of market value, with stock photos and a story about a quick sale, are counterfeit listings.
If you already bought one
Stop wearing it and preserve the listing, messages, and payment records. Ask a watchmaker who services IWC watches to document the findings in writing, then use that documentation for a payment dispute. Counterfeit trademarked goods are illegal to import into the United States, and shipments can be seized, so reporting the seller to the platform and to customs authorities is a reasonable step. Do not resell the watch. Passing it on creates the same problem for the next buyer.