The fastest way to settle whether an IW371646 is genuine is a serial and movement check performed by an IWC service point or an independent watchmaker who opens the caseback. Dial photographs alone do not prove anything, because the better counterfeits copy the dial face closely. The checks below are ranked by how hard they are to fake: movement architecture first, then case finish and dimensions, dial printing and subdial layout, caseback engraving, and finally the paperwork trail. This is a screening guide for buyers who want to avoid a counterfeit, not a sourcing guide for one.

1. IWC Service Verification: The Top Pick

An authorized IWC service point can cross-check the caseback serial and reference against factory records, and a watchmaker who opens the watch can identify a non-genuine movement or swapped parts on sight. This is the only check that produces a defensible answer rather than an opinion.

• Pros: definitive serial cross-check; a written service estimate that names non-genuine parts; useful evidence if you need to reverse a payment or open a dispute.
• Cons: costs money and takes days or weeks; a service center may decline to work on an obvious fake, which is itself an answer but leaves you without paperwork; it does not help at a same-day meetup.

Best for: any private purchase where the seller cannot show an original receipt, and any deal where the price sits far below the market for the reference.

2. Movement Architecture

The Portugieser Chronograph family in this reference generation runs an in-house column-wheel chronograph calibre with a two-subdial layout. Counterfeits almost always use a Valjoux 7750 base or an Asian chronograph movement, and those leave traces: a date ring or date wheel that should not be present, a chronograph pusher feel that is stiff or grindy, a rotor with shallow engraving, and finishing that looks printed rather than cut. Ask for a clear photograph of the movement through the display back and compare the shape of the bridges, the position of the balance, and the engraved calibre number against IWC's published specification for the exact reference.

• Pros: movement layout is expensive to counterfeit correctly; a mismatch is hard to explain away.
• Cons: hard to judge from a phone photo; some sellers use genuine movement shots from other listings.

Best for: remote purchases where you must decide from images before shipping.

3. Case, Crystal, and Finish

Genuine cases are machined to tight tolerances, so the lug profile flows into the caseband without a visible seam and the crown sits flush. Common tells on a fake include a case that measures a millimetre or two off the published diameter or thickness, sharp lug edges, lugs that sit at the wrong angle, a crown that wobbles, and a crystal with visible distortion or a weak anti-reflective coating that washes the dial out under a phone light. Weigh the watch if you can; a counterfeit usually feels lighter or oddly balanced.

• Pros: measurable with calipers and a scale; no disassembly needed.
• Cons: case refinishing on a genuine watch can mimic some of these flaws; published dimensions must be confirmed for the exact reference.

Best for: in-person inspections, where you have the watch in hand.

4. Dial, Hands, and Printing

Check the subdial arrangement against the published layout for the reference. Many counterfeits add a date window or shift the subdial positions because they are built around a movement that does not match the original. Look at the brand lettering under magnification: genuine printing has crisp edges and consistent stroke weight, while counterfeit printing looks slightly fat, fuzzy, or unevenly spaced. Applied indices should be set straight, hands should be cleanly finished with no burrs, and the small seconds hand should sweep without stutter.

• Pros: visible in ordinary listing photos; cheap to check with a loupe.
• Cons: dial swaps exist on genuine watches, so a correct dial does not prove the whole watch.

Best for: a first-pass filter before you spend money on any deeper check.

5. Caseback Engraving and Serial Numbers

Genuine engraving is deep, evenly spaced, and consistent in font. Counterfeit engraving is often shallow, uneven, or lasered with a slightly different typeface. Photograph the serial and reference, then have the numbers checked rather than searching forums, where old and partly wrong information circulates.

• Pros: quick and objective; serial can be verified independently of the seller.
• Cons: counterfeiters copy real serial numbers from listings, so a plausible number alone proves nothing.

Best for: every transaction, including ones that look clean.

6. Papers, Boxes, and Seller Behavior

Documentation is supporting evidence, not proof. Boxes and cards are widely reproduced, and a genuine box tells you nothing about the watch inside it. Weigh the story instead: a seller who refuses a service-center check, insists on a bank transfer, has no service history, prices the watch far below the market, or pressures you to decide the same day is telling you something.

• Pros: costs nothing; catches pressure tactics before money moves.
• Cons: honest sellers also lose paperwork, so missing papers alone is not disqualifying.

Best for: the final decision, always alongside a physical or movement check.

7. Which Route Fits Your Situation

Buying in person, meeting a private seller

Bring calipers, a scale, and a loupe. Compare case dimensions and weight to IWC's published figures, confirm the dial layout, and make the sale conditional on a service-center check within a set number of days.

Buying remotely

Require a short video showing the chronograph starting and stopping, the running seconds, and the movement through the caseback. Then insist on a third-party authentication service that holds the funds until it clears.

You already own it and suspect it is fake

Take it to a service point. If the watch is counterfeit, stop wearing it and stop trying to service it, and if you paid by card or a protected payment method, start the dispute with the service report in hand.

Bottom Line

No single photograph proves an IW371646 is genuine, but a service-center serial check plus an opened-caseback movement inspection comes close. Work the criteria in order: movement, case, dial, engraving, paperwork, seller. If a seller blocks the verification step, treat that as the answer and walk away.