Short answer: an IW371643 is unverified until the seller proves three things, the reference engraving between the lugs, the caseback serial, and a service record that matches both. The IW371643 belongs to the IWC Portugieser Chronograph family, and that family is one of the most copied lines in the catalog. The outside is easy to imitate. The inside is where fakes fall apart, so any seller who will not open the case is a seller you walk away from.
Before you ask anything
- Request photos in natural daylight, not studio light, of the dial, the caseback, and all four lugs.
- Ask for a short video with the chronograph started, stopped, and reset.
- Know the fair market range for the exact reference so an unusual price stands out on its own.
The questions that settle it
- Ask the seller to read the reference number engraved between the lugs and send a close photo of it. Compare the format against IWC's own published data for the reference.
- Ask for the case number from the caseback and request that it appear in the same photo as the reference engraving. Two separate photos prove nothing.
- Ask which calibre sits inside and whether the movement has ever been serviced. A Portugieser chronograph should show a signed rotor and finishing when opened.
- Ask for the original warranty card, booklet, and box, then check that the serial on the card matches the caseback.
- Ask whether the seller will accept a return if an authorized watchmaker or IWC client service declines to authenticate the watch. Get that in writing.
Physical checks on arrival
- Inspect the engraving between the lugs under a loupe. Laser etching looks shallow and uneven next to factory stamping.
- Start the chronograph and watch the central seconds hand. It should start without a visible jump and reset to zero cleanly.
- Check the subdial spacing and the printing weight of the text on the dial. Fakes blur thin type and misalign subdial centers.
- Feel the crown action. Gritty threading and wobble point to a swapped or aftermarket crown.
- Weigh the watch against the published specification for the reference.
Red flags that end the conversation
- The seller refuses macro photos of the caseback or lugs.
- The reference number returns no match in official records.
- The price sits far below every comparable listing.
- Papers exist but the serial does not match the watch.
- The seller pushes a fast sale through a payment method with no recourse.
If the watch fails the check
Stop the transaction and keep every message and photo. Report the listing to the platform. If money already changed hands, file a dispute with your payment provider the same day and take the watch to an authorized watchmaker for a written assessment. A documented opinion from a professional carries far more weight than an online verdict.