Short answer

A fake IW371747 is a watch sold as an IWC Portugieser Chronograph of that reference when the case, dial, or movement did not come from IWC. No single photo settles the question. You confirm authenticity by matching the reference and case serial to the paperwork, measuring the case with calipers, inspecting the dial print under magnification, and having a watchmaker open the back to identify the movement.

Before you start

  • A 10x loupe or a macro lens
  • Digital calipers and a small scale
  • The official spec sheet for the exact reference from IWC
  • Clear photos of the dial, case back, crown, and clasp
  • Access to a watchmaker or an IWC service point

Step-by-step check

  1. Write down the full reference and the case serial from the seller's listing, then pull the same reference from the manufacturer's product page.
  2. Compare the reference digits one at a time against the spec sheet, since a single wrong digit points to a different watch.
  3. Lay the watch on a flat surface and photograph the case back engraving at close range.
  4. Measure the case diameter, thickness, and lug width with calipers and compare each number to the spec sheet.
  5. Weigh the watch on the scale and compare the figure to the published weight for that reference and configuration.
  6. Study the dial print at 10x and check letter spacing, font weight, and the alignment of the brand name above the subdials.
  7. Check the position of each subdial and confirm it matches the reference layout, not the layout of a cheaper base movement.
  8. Look at the date window under magnification and check the aperture shape, print quality, and centering of the number.
  9. Press each chronograph pusher and listen for a crisp, even click, then reset and confirm the hands return to zero.
  10. Turn the crown through several full rotations and feel for consistent resistance with no grit or wobble.
  11. Inspect the clasp, buckle, and strap markings, then compare the stamping depth and font to known photos.
  12. Ask a watchmaker to open the case back and identify the movement by caliber, finish, jewel count, and rotor engraving.
  13. Send the case serial and reference to an IWC service point or authorized dealer and request written confirmation.
  14. Request the original invoice, warranty card, and box, then check that the serial on each item matches the watch.

Red flags that point to counterfeit

  • A price far below the current used market for the reference
  • A seller who refuses to share the case serial or let a watchmaker open the watch
  • Dial print with uneven spacing, soft edges, or the wrong font weight
  • Subdials in the wrong position or hands of the wrong length
  • Case dimensions that sit outside the published tolerance
  • Paperwork with a serial that does not match the case
  • A generic third-party movement where an in-house chronograph caliber belongs

If you already bought one

  1. Stop wearing the watch and keep it in the condition you received it.
  2. Save the listing, messages, payment receipt, and shipping label.
  3. Get a written opinion from a watchmaker or service point.
  4. Open a dispute with your payment provider or card issuer using that written opinion.
  5. Report the sale to the platform and, where relevant, to customs or consumer protection authorities.

What the reference number tells you

The IW371747 reference covers one specific dial, case, and strap combination. Buyers who skip this check often compare a counterfeit against the wrong photos and miss obvious differences. Match the reference first, then judge the watch.