Quick answer

The IW371431 is a reference in the IWC Portugieser Chronograph line: a 40.9 mm stainless steel case, a bi-compax dial with subdials at 12 and 6, no date window, and the IWC caliber 79350 automatic chronograph inside. A fake copies the silhouette and fails on the details. The three fastest tells are the subdial layout, the crystal coating, and the movement visible through the display back.

Expect a counterfeit to run an Asian 7750 clone, a quartz module, or a decorated low-grade automatic. Each one leaves physical evidence you can measure and see.

Prerequisites

  • 10x jeweler's loupe
  • Digital caliper with 0.1 mm resolution
  • Bright, diffused light and a clean white surface
  • High-resolution photos of a known-genuine IW371431 for side-by-side comparison
  • A watchmaker willing to open the caseback, or a caseback tool if you own the watch

Steps

  1. Measure the case with the caliper at the bezel. A genuine Portugieser Chronograph sits at roughly 40.9 mm across, excluding the crown, and about 12.3 mm thick. Copies often land at 42 mm or 43 mm because they reuse a generic case.
  2. Check the dial for a date window at 3 o'clock. The IW371431 does not have one. Any aperture cut into the dial at the 3 position is disqualifying on its own.
  3. Count and locate the subdials. You want two: small seconds at 6 and elapsed chronograph minutes at 12. Three subdials, or a subdial at 9, points to a different movement.
  4. Study the printed text under the loupe. Genuine dial printing is crisp with hard edges. Fakes show thickened serifs, uneven spacing between letters, and ink that bleeds into the dial finish.
  5. Examine the applied numerals and hands. The indices are applied metal, not printed, and sit flat against the dial. The hands are leaf-shaped with polished facets that catch light along a sharp center ridge. Stamped hands look flat and dull.
  6. Angle the crystal toward a light source. The sapphire has an anti-reflective coating that produces a faint reflection. A strong blue or purple flare across the whole crystal is a common sign of an aftermarket coated glass.
  7. Press the chronograph pushers. Genuine 79350-based chronographs start with a firm, short throw and a clean click. Mushy travel, a grinding feel, or a pusher that sticks indicates a cheap chronograph module.
  8. Watch the chronograph seconds hand sweep. A quartz fake ticks in one-second jumps. A mechanical copy may sweep, so confirm the movement through the caseback before you accept it.
  9. Inspect the caseback engraving. Reference numbers, the IWC Schaffhausen name, and the case metal markings are laser-cut or stamped with sharp walls. Engraving that looks shallow, filled with dark paint, or misaligned is a warning.
  10. Look through the display back at the movement. You should see a rotor with IWC engraving, Geneva stripes on the bridges, blued screws, and perlage on the base plate. A plain rotor, printed decoration, or visible plastic parts ends the inspection.
  11. Weigh the watch. A steel Portugieser Chronograph on a leather strap falls in a narrow band. A noticeably light watch suggests a plated brass or hollowed case.
  12. Cross-check the serial number against the warranty card, the caseback, and any service paperwork. Numbers that do not match across all three documents are a hard stop.
  13. Have a watchmaker time the movement on a timing machine. A genuine caliber holds steady rate and amplitude. A copy usually shows erratic readings and low amplitude.

Red flags on the dial

Print quality, numeral thickness, and subdial hand length are where copies spend the least effort. On the IW371431, the small seconds hand reaches close to the minute track. On many fakes it stops short. The rail track around the outer edge should be a continuous printed scale, not a series of broken marks.

Movement and caseback

The caliber 79350 is a modified Valjoux 7750 base built to IWC specification. Fakes rarely reproduce the finish or the rotor engraving. If the seller refuses to let a watchmaker open the back, treat that as a refusal to be verified. No legitimate sale depends on secrecy.

Paperwork and serials

Blank warranty cards, cards filled in with a different pen from the box label, and sellers who ship only a watch and no documents are common in the counterfeit market. Ask for the original purchase receipt and the dealer name. Then call the dealer.

What to do if you already own a fake

Stop wearing it and stop reselling it. Selling a counterfeit watch is a trademark violation in the United States and most other markets, and it exposes you to civil liability. Report the seller to the platform where you bought it and to the brand's anti-counterfeiting contact. If you want the Portugieser look, buy a genuine reference from an authorized dealer or a documented pre-owned listing, or choose an honest entry-level chronograph instead of a copy that will fail within a year.