Quick answer

The IW371431 is an IWC Portugieser Chronograph reference. The genuine watch has a 40.9 mm steel case, a thickness near 12.3 mm, a 20 mm lug width, caliber 79350 with 44 hours of power reserve, and a 30 m water resistance rating. A replica passes when diameter, thickness, subdial layout and date position match those numbers. Most fail on thickness, and on the running seconds hand at 6.

The -31 suffix marks one dial and strap combination. Confirm that combination against catalog photos first. Sellers mix dial colors under the same reference.

Case and thickness

Measured with a caliper, a pass reads 40.8 to 41.0 mm across the bezel and 12.2 to 12.6 mm thick. Replicas built on a stock 7750 with a module run 13.5 mm and up. That extra 1.5 mm shows at the wrist and under a shirt cuff. Lug tips should curve down. Flat lugs and a thick caseback are the two signs of a low grade build.

Dial and subdials

Genuine layout: 30-minute totalizer at 12, small seconds at 6, date at 3. Hands are leaf shaped. Printing is thin and black. Check four points: subdial hand length, the date window cut, the gap between subdial and center, and the space between the 12 marker and the minute track. Crooked applied numerals and thick printing are the common faults.

Movement

The original runs caliber 79350, a modified Valjoux 7750 at 28,800 vph. Replicas use an Asian 7750 with a transfer gear to move running seconds to 6. That gear train adds drag. Ask for a timed rate test. Accept plus or minus 15 seconds a day on a fresh unit. Reject any watch whose chronograph hand fails to snap back to 12 after reset.

Caseback and hardware

The back is sapphire with an engraved rotor and the reference between the lugs. The crown is signed. Chronograph pushers should start, stop and reset with a firm click and no grit. The watch is rated to 30 m, so treat a replica as splash resistant only. Seals on replica cases are not tested at the factory.

Checks before payment

  • Caliper reading of diameter and thickness.
  • Subdial hands return to zero after a full chrono cycle.
  • Date flips at midnight, not at 11:30 pm.
  • Caseback engraving legible under a loupe.
  • Weight of 70 to 90 g on a leather strap.
  • Buckle signed, stitching even.

Care

The 7750 family needs service every 5 to 7 years. The seconds-at-6 version needs it sooner. Do not set the date between 9 pm and 3 am. Keep the watch dry and wind it once a month when it sits in a box.