A genuine IW371745 is an IWC Portugieser Chronograph built in stainless steel. Copies of this reference are common, and most of them fail on the same three points: case finishing, dial printing, and the movement visible through the sapphire caseback. If any one of those is wrong, the watch is not genuine, regardless of what the seller claims.

What you need before you start

  • A 10x loupe
  • Calipers or a ruler marked in half millimeters
  • Photos of a confirmed genuine example shot at the same angle
  • A bright light you can tilt to rake across the surface

Case and lugs

  1. Measure the case across the bezel and compare the result with the official specification for the reference. Copies are often half a millimeter to a full millimeter off, and the difference shows at the lugs.
  2. Run a fingernail along the top edge of each lug. Genuine IWC lugs are crisp and even. Fakes feel soft, wavy, or rounded because the case was cast and polished rather than machined.
  3. Tilt the watch under the light and follow the line where the polished bezel meets the brushed case flank. A genuine watch holds one clean, straight line. A copy shows a wandering border and polish bleeding onto the brushed surface.

Dial and hands

  1. Inspect the printing at 10x. Genuine text is sharp, level, and evenly spaced. Fake dials show thick, fuzzy letters, uneven paint fill, and a brand name that drifts off center.
  2. Check the subdial rings for concentricity. On a genuine dial the rings sit exactly on center; on copies they shift by a fraction and the gap changes as you move around the ring.
  3. Compare hand length against the markers. Genuine hands reach the outer track. Fake hands stop short or overshoot.
  4. Look at the date aperture if the reference has one. The genuine window is beveled and the numerals are aligned with the frame. A copy shows a raw stamped opening.

Chronograph function

  1. Press the start pusher. A genuine chronograph engages with a firm, distinct click. Vague or spongy travel points to a cheap module.
  2. Run the chronograph for a minute, then reset. The seconds hand should snap to zero in one motion and land exactly on the twelve marker. A fake sweeps back slowly or stops a degree or two off.

Caseback and movement

  1. Study the engraving on the caseback. Genuine laser engraving has uniform depth and clean letterforms. Copies show shallow, inconsistent cuts and misspelled or misaligned text.
  2. Hold the watch up to the light through the display back. Check for perlage, blued screws, and engraved bridges. Generic clone movements show stamped plates, plastic spacers, and rotor decoration that does not match IWC finishing.
  3. Have a watchmaker open the case and read the caliber number. This is the only check that settles the question with certainty.

Price and paperwork

A price far below the secondary market for this reference is the strongest single signal. So is a serial number that appears in more than one listing, or a seller who cannot show the warranty card matching the case number. Cards and boxes are easy to copy, so treat paperwork as supporting evidence and never as proof on its own.

If the watch fails one check, stop. Fakes rarely fail only one.