Quick answer
Most watches sold as an IW371790 are not tied to a reference that IWC publishes. Before you judge the watch, judge the number. If the reference does not appear in IWC's own model list, the listing is mislabeled or built around a string that does not exist, and no amount of dial inspection fixes that. If the number does check out, authentication comes down to five measurable things: case dimensions, crystal coating, dial printing under magnification, movement finishing, and the paperwork trail.
Before you start
- A 10x loupe or a phone macro lens
- A digital caliper and a kitchen scale that reads to 1 g
- Saved official photos and the spec sheet for the exact reference
- Permission from the seller to photograph the movement or open the caseback
How to authenticate an IW371790
- Confirm the reference exists. Search the exact string on IWC's official model list. If nothing returns, ask the seller for the reference printed on the caseback and on the warranty card. A number that appears nowhere in the catalog is the first and often the only answer you need.
- Save a baseline. Download the official product photos and the specification sheet for the confirmed reference. Every later comparison runs against those files, not against memory and not against another listing.
- Measure the case. Use a digital caliper across the bezel at 3 and 9 o'clock, then record thickness from caseback to crystal. Compare both figures to the spec sheet. A case that reads 2 mm wide or 2 mm too thick is not a finishing error, it is a different case.
- Weigh the head. Set the watch on a kitchen scale with the strap or bracelet removed. Note the figure. Builds that use a hollow mid-case or a lighter movement sit outside the genuine range, and the gap shows once you hold the real number.
- Examine the crystal under a lamp. Tilt the watch so a light source crosses the glass. Genuine IWC sapphire carries an anti-reflective coating on both faces and gives a faint blue-violet sheen. A strong purple or green flare, or coating on the outside face only, points to aftermarket glass.
- Inspect the dial printing with a 10x loupe. Check the brand name, the town line, and the depth rating. Genuine printing is crisp, evenly spaced, and flat against the dial. Fuzzy edges, uneven letter spacing, or ink that sits proud of the dial are defects a real dial does not have.
- Study the hands and subdials. Genuine hands are diamond-polished with straight edges and no burrs. Look at the subdial rings through the loupe: they should be cut into the dial, not printed on top, and the hands should sit parallel to the surface.
- Read the caseback engraving. Check the reference, the serial, and the font weight. Genuine engraving cuts deep with consistent depth and clean interior corners. Shallow stamping with rounded corners is a warning sign.
- Open the caseback or request movement photos. Compare the finishing to the calibre named on the spec sheet: rotor engraving, bridge decoration, jewel count, and shock system. A rotor stamped with an IWC logo over an off-the-shelf base is the clearest tell there is.
- Test the chronograph. Start, stop, and reset the chronograph three times. Pushers should travel with even resistance and the reset should snap back to zero in one motion. Mushy pushers and hands that settle off zero mean the module is not the original part.
- Check the paperwork. Match the serial on the caseback to the warranty card and the box label. Missing or mismatched papers do not prove a fake by themselves, but they mean you should treat the watch as undocumented and price it that way.
Red flags that end the conversation
- A reference string that does not appear in IWC's catalog
- A seller who refuses a caseback photo
- Case dimensions that miss the spec sheet by more than a fraction of a millimeter
- Dial printing that fails a 10x loupe
- Serials on the caseback and card that do not match
What graders check first
Most bad watches are caught on dimensions and glass, not on the movement. Case diameter, thickness, and crystal coating cost a few minutes to measure and are hard to fake at scale. Dial printing is the next filter, because genuine pad printing holds a sharp edge under magnification while copy work frays. Movement finishing is the last check and the one that settles the question, so make it a condition of the sale rather than a favor you ask for after money changes hands.