Short answer

IWC reference numbers use the letters IW plus six digits. One example is IW356501, a Portofino Automatic at 40 mm. The string "IW938" holds four characters after the letters. It does not match that format. No Portofino in the IWC catalog carries it. Listings that print "IW938" come from sellers, not from IWC. Sellers use short codes to label a batch. The code groups stock for the seller. It is not a factory number.

One note on history. IWC used four digit reference numbers before 2000, with no IW prefix. The Portuguese ref 5251 is one case. I found no Portofino with reference 938 in public IWC material. Treat "IW938" as a seller label until a primary source shows otherwise.

How IWC reference numbers work

Since the 2000s IWC writes eight characters: IW plus six digits. The first four digits name the model family and the movement group. The last two digits name the version. Version covers dial color, strap, and case metal. IW356504 and IW356505 share a case and a caliber. They differ in strap.

Portofino numbers that exist

  • IW3565, Portofino Automatic, 40 mm, caliber 35111
  • IW4581, Portofino Automatic 37, 37 mm
  • IW3910, Portofino Chronograph, 42 mm
  • IW4590, Portofino Automatic Moon Phase 37
  • IW5164, Portofino Hand-Wound Moon Phase, 45 mm

A base number runs with two trailing digits. IWC's site lists the current set. Reference databases list older ones. Both search boxes are free.

What shows up on replica casebacks

Replica makers copy engraving from photos. Four errors repeat.

  1. Digit count. "IW938" carries three digits. IWC model numbers carry six.
  2. Wrong family. A Pilot or Aquatimer number stamped on a Portofino body.
  3. Repeated serials. Ten units with one serial number.
  4. Missing text. Real casebacks carry the reference, the serial, the model name, and the Swiss Made stamp.

Checks on a Portofino listing

  1. Count the characters. Eight for a modern IWC number.
  2. Search the full number. IW356501 returns a match. IW938 returns nothing.
  3. Match the case size. Automatic is 40 mm. Chronograph is 42 mm. Hand-Wound is 45 mm.
  4. Read the movement name. Portofino Automatic uses caliber 35111. A printed number on a generic rotor does not prove the caliber.
  5. Measure thickness. The Portofino Automatic is 9.2 mm thick. Copies run 11 mm to 13 mm.

Legal status in the US

Counterfeit watches are illegal to import into the United States. U.S. Customs and Border Protection can seize a shipment and destroy it. The same rule covers sales inside the country. A buyer gets no warranty and no service path from IWC.

Bottom line

"IW938" is not an IWC Portofino reference. Use the eight character format to test any listing. If a seller cannot name the six digit number, the listing is unverified.