Short answer: IW371660 is an IWC Portugieser Chronograph reference, and the fastest way to identify a fake is to check the caseback engraving and the movement together, because the two have to agree with each other. Of the checks below, a movement inspection by a watchmaker who routinely services IWC is the top pick, and it is the top pick because it is the only method a well-built counterfeit cannot pass. The criteria that separate a useful check from forum folklore are reproducibility (you can repeat it on another example), specificity (the detail has to be unique to that reference), and independence (you can verify it without trusting the seller). Photo comparisons alone fail the first test; paperwork alone fails the third.

1. Caseback engraving and reference markings

Genuine IWC casebacks carry laser engraving that is shallow, even, and consistent in stroke weight: the reference number, the case serial, the material code, and the country of origin marking. Counterfeit cases are usually stamped, and the cut sits deeper and wider, with spacing that drifts across the line of text. The practical move is to compare the case serial on the back with the serial on the warranty card and in any service records. A mismatch is a hard signal. A match proves nothing, because serials get copied off listing photos.

  • Pros: free, repeatable, works from seller photos before you travel, needs no tools.
  • Pros: a serial mismatch is one of the few single tells that justifies walking away immediately.
  • Cons: a good fake will match the paperwork it ships with.
  • Cons: it tells you nothing about the movement, which is where most money is saved.

Use it when: you are screening an online listing and want a cheap reason to discard the obvious junk before spending time on the serious candidates.

2. Movement and calibre

Confirm the calibre and its architecture against IWC's published specification for that reference before you inspect anything, because reference numbers in the Portugieser Chronograph line sit close together and buyers mix them up. IWC fits in-house automatic chronograph calibres in this family. Counterfeits commonly hide a generic or cloned chronograph movement behind a decorated rotor and an engraved caseback. What you look for is the calibre number stamped on the movement, the finishing on the bridges, and whether the movement number agrees with the case. A watchmaker opens the back in minutes.

  • Pros: decisive, and the one check that a counterfeit cannot fake at a profit.
  • Pros: a service quote from the same watchmaker gives you a rough idea of running costs before you buy.
  • Cons: needs a watchmaker, so it does not work on a listing you are only browsing.
  • Cons: a careless opening can mark the caseback, and that damage follows the watch.

Use it when: money is about to change hands. Make it a condition of sale, in writing, or walk.

3. Dial printing, hands, and subdial layout

Dial work is where cheap counterfeits give themselves away to the naked eye. Genuine printing sits crisp against the dial surface, with even weight in the rail track and clean edges on every marker. Applied numerals on a fake often sit proud, or lean, or catch light in a way the genuine article does not. Hands are another weak point: look for flat stampings instead of faceted or polished forms, and for white paint that fills the wrong portion of a hand. On references that use twin subdials, check which subdial counts what, and check that any aperture, hand shape, or marker matches the official photographs of that exact reference.

  • Pros: no tools, no cost, and usable on clear high-resolution photos.
  • Pros: catches the mid-tier fakes that pass a caseback glance.
  • Cons: lighting and phone cameras hide real flaws, so photos mislead in both directions.
  • Cons: later dial swaps on genuine watches complicate the comparison.

Use it when: you are comparing three or four listings side by side and want to narrow the field quickly.

4. Case, crystal, and finishing

Measure the case against IWC's published dimensions rather than trusting the listing. Counterfeit cases are frequently a fraction off in diameter or noticeably thicker, and thickness is easy to feel on the wrist even when a photo hides it. Run a finger along the lug edges: genuine cases transition from brushed to polished surfaces on a clean line, while counterfeit cases show rounded or wavy transitions where a polishing wheel went too far. Check the crystal for an anti-reflective coating that looks purple or blue under a desk lamp, and for a cyclops or distortion that should not be there.

  • Pros: thickness and lug feel are hard for a counterfeit to correct at low cost.
  • Pros: calipers settle arguments that photos cannot.
  • Cons: a genuine watch that has been polished can show the same soft edges.
  • Cons: published dimensions vary slightly across production batches of the same reference.

Use it when: you have the watch in hand and want a second opinion that does not depend on the seller's story.

5. Papers, provenance, and price context

Boxes, cards, booklets, and hang tags are copied in bulk, so they carry little weight on their own. What carries weight is a documented chain: a dated purchase receipt with a dealer name, service records with matching serials, and an owner who can explain the history without hesitation. Price is the loudest signal of all. When an IW371660 is offered far below the going rate for that reference in that condition, the discount is usually the cost of the problem you have not found yet. Counterfeiting is illegal in most markets, and passing a counterfeit off as genuine is fraud wherever you are.

  • Pros: receipts and service history are cheap to check and hard to fake convincingly over several years.
  • Pros: a seller who welcomes an inspection is telling you something useful.
  • Cons: paperwork gets lost on genuine watches, so absence is not proof of anything.
  • Cons: pressure selling and deadline tactics are a red flag that outranks the documents.

Use it when: you are buying from a private seller rather than a dealer with a return policy.

If you already own a suspicious IW371660

  1. Photograph the caseback, the dial, and the movement if you have already had the back opened.
  2. Take it to a watchmaker who services IWC and ask for a written opinion, not a verbal one.
  3. Contact the seller in writing, keep the message thread, and ask for a refund before you name the problem publicly.
  4. If the seller refuses and you paid by card or a protected payment method, open a dispute with the evidence you collected in step one.
  5. Do not resell it as genuine. That turns a bad purchase into an offence.

The short version: verify the calibre for the reference first, inspect the movement second, and treat everything else, engraving, dial, dimensions, and paperwork, as supporting evidence. Any single check can be beaten. The set of them together is what tells you whether an IW371660 is the watch it claims to be.