Short answer

IWC does not show IW371787 in the reference lists published on its site or in the catalogs I could check. The 3717 block belongs to the Portugieser Chronograph Classic family in older IWC material. A watch sold as an IW371787 is one of three things: a reference the seller invented, a number from an internal list, or a counterfeit case stamp. Ask IWC or an authorized dealer to confirm the number before you send money.

What an IWC reference number does

IWC numbers start with IW and carry six digits. The first four digits name the model. The last two name the dial color, case metal, or retail channel. Two watches with the same four digits can sit $4,000 apart in price. A seller who cannot explain the last two digits is guessing.

Movement checks

The Portugieser Chronograph Classic uses IWC caliber 89361. Published figures: automatic winding, 68 hours of power reserve, 28,800 vibrations per hour (4 Hz).

Most fakes in this size class carry a Chinese copy of the Valjoux 7750 or a quartz module.

  • Running seconds at 9 o'clock point to a 7750 layout. The Portugieser layout puts running seconds at 6.
  • Rotor engraving on fakes is shallow, and the letter spacing is uneven.
  • A quartz tick at the running seconds subdial rules the watch out. IWC built no quartz Portugieser Chronograph Classic.
  • Case back screws on fakes strip after one or two openings.

Case and dial checks

Published size for the Portugieser Chronograph Classic: 42 mm across, about 14.5 mm thick. A steel version on a leather strap lands near 90 to 100 g on a kitchen scale. Fakes in this family run 15 to 30 g light because the case walls are thinner.

  • Print. Genuine IWC and SCHAFFHAUSEN text sits on one baseline with tight letter spacing.
  • Applied numerals sit flush on the dial. Fakes show glue shadows or crooked indexes.
  • Sapphire crystal carries anti-reflective coating on both faces. A fake shows a blue or purple tint under a lamp.
  • Pushers. Genuine chronograph pushers have a firm start and a clean reset at the first press.

Price checks

Used Portugieser Chronograph Classics trade near $5,000 to $10,000 USD on the secondary market, more for gold and for full sets. Replica versions sell for $80 to $700. A $300 ask for a steel IWC chronograph with papers is a fake or a stolen watch. There is no third option.

Red flags

  • Seller refuses photos of the case back and the movement.
  • The reference number on the case back does not match the number in the listing.
  • Serial number is engraved over a brushed surface with raised edges.
  • Box and papers arrive without a matching serial.
  • Payment by gift card, crypto, or wire to a personal account.

How to verify a watch

  1. Write down the full reference and serial from the listing.
  2. Ask IWC customer service or an authorized dealer to confirm the reference exists and that the serial matches the model.
  3. Request movement photos with the case back off, taken on a plain background.
  4. Have a watchmaker with an IWC parts account open the case and read the caliber stamp.
  5. Buy from a seller who accepts a return window of at least 7 days.

What this guide cannot confirm

IWC does not publish every reference, and boutique-only numbers exist. IW371787 may be one of those. It may also be a number a counterfeiter stamped on a case. Only IWC can settle the question. Treat a listing without written confirmation as unverified.