Short answer
A "fake IW371613" is a counterfeit IWC Portugieser Chronograph wearing a genuine reference number. The number is the whole point. It locks in a 41 mm case, a two-register chronograph layout with elapsed minutes at 12 and running seconds at 6, and IWC's in-house calibre 69355 sitting behind a sapphire display back. Fakes copy the silhouette and lose the details. The details are where you catch them.
Two things are worth saying up front. Counterfeit versions of this reference exist at every price tier, from a mall copy to a nine-hundred-dollar "super clone." And the caseback is your best tool. Almost no fake for this reference gets the movement right, and the ones that come close give themselves away in the finishing.
Why the exact reference matters
IWC assigns a separate reference to each dial, strap, and case-metal combination in the Portugieser Chronograph line, and the trailing digits split those variants apart. So the first check is boring and it comes before anything else: confirm what IW371613 should look like using IWC's own product data. Save the official photos. Then compare the seller's images to them. If the dial, hands, or case metal do not match the reference being quoted, stop. A seller who is sloppy about the reference is not a seller you want to trust with a wire transfer.
Case and finish
The Portugieser case has a specific mix of finishes. Lugs are straight-grained on their flanks, the bezel edge carries polish, and the transition between the two is sharp but not harsh. Fakes tend to go one of two ways: over-polished to the point that lug edges look rounded and soft, or left with machining marks the real thing never shows. Check the crown too. It should be signed, well seated, and turn with light, even resistance. On cheap copies the crown is undersized and the engraving is shallow enough that you can feel it disappear under a fingernail.
Dial printing and subdials
Look at the dial under a loupe or a phone macro lens. Applied numerals should be faceted and sit proud of the surface. Leaf hands should be thin with a clean polish line down the middle. The subdial rings are recessed and finished with a fine snailed texture, not stamped flat. Printing is crisp at 10x magnification, including the small text near the date or the outer track. Fakes usually fail here first: fuzzy printing, numerals that sit too flat, hands that are a hair too thick, and subdial rings with no depth at all.
The movement and caseback
This is the decisive test. A genuine example shows a column-wheel chronograph movement through the back, with Geneva stripes, perlage on the plates, and engraved text on the rotor. Ask the seller for a sharp, angled photo in daylight, not a dim phone shot. A decorated Valjoux-style movement or a quartz module is an immediate no. Then ask for a video of the chronograph starting, stopping, and resetting to zero. A real column-wheel action feels crisp and light. Pushers that grind or feel mushy are a warning sign you can detect from a video if the seller talks while pressing them.
Strap, buckle, and packaging
Straps get swapped, so do not judge a watch on leather alone. Judge the buckle. Stitching density is even, the tang profile is correct, and the engraving is deep and clean. On the packaging side, a genuine warranty card carries a serial that matches the caseback, and the booklet printing is sharp, not a photocopy with gray text. "Box and papers" is not proof of anything by itself, but mismatched paperwork is proof of a problem.
A price reality check
Steel Portugieser Chronographs trade on the secondary market for well under retail, and that discount is easy to research. What does not exist is a genuine, complete example at ten percent of retail from a stranger on a classifieds site. When the price is the tell, trust the price.
Before you pay
- Pull the official reference images and specs from IWC and compare them to the listing, line by line.
- Request a video of the chronograph running and resetting to zero.
- Get a clear photo of the caseback serial and the warranty card, then check that the two match.
- Ask for a movement photo shot at an angle, in daylight, with the rotor visible.
- At an in-person meeting, bring calipers. Measure the case and feel the weight against published specs.
- Pay with a method that gives you recourse. Wire transfers and gift cards do not.
If a seller refuses any of these, that refusal is your answer. A genuine seller with a real watch has nothing to lose by showing it.