The top pick for this question is not a seller but a sequence of checks: caseback stamp first, sub-dial layout second, movement family third. Run those three in order and most fake IW371745 listings sort themselves out before you study a dial photo. The sections below take each criterion in turn, with what the check catches and where it stops working.

What the reference is, and what "fake" covers

IW371745 sits inside the IWC Portugieser Chronograph family: two registers, chronograph minutes at 12 o'clock, running seconds at 6 o'clock, no date window, a slim bezel and a case in the region of 41 mm. The line has run across more than one generation, so the movement underneath can be either the older 79350 (a modified Valjoux 7750 base) or the in-house 69355. Confirm which generation your reference belongs to before you compare movements.

Replica output for this layout runs from a printed-dial quartz watch to a mechanical clone with a passable case profile and a decorated but wrong movement. Price bands track that range, and a seller who will not say which grade is on offer is telling you something useful.

Criterion 1: case profile and lugs

The Portugieser silhouette is long, flat and thin-bezelled, and that shape is harder to fake than a dial.

  • Pros: a side-on or three-quarter photo shows lug curve, bezel step and crown size in one frame, so outline errors surface fast.
  • Pros: case thickness and lug-to-lug are measurable numbers, and a clone that is 0.5 mm thick shows in a wrist shot.
  • Cons: the contrast between polished flanks and brushed surfaces disappears under ring lighting.
  • Cons: a case can be right in outline and wrong in weight and balance on the wrist.

Criterion 2: dial, printing and sub-dial spacing

This is the strongest single tell on a fake IW371745. The two registers sit on a vertical axis, and the applied numerals at 12 and 6 are cut by them in a specific way.

  • Pros: the way the registers intersect the numerals is costly to reproduce and cheap to check.
  • Pros: print weight on the track and the markers is visible in a normal, unfiltered photo.
  • Cons: dial variants changed by generation, so a "wrong" dial may simply be a different reference.
  • Cons: filtered and retouched listing photos hide print quality and flatten applied markers.

Criterion 3: movement and chronograph action

The caseback on this family is closed, so the movement is the check most buyers skip and the one that decides the grade.

  • Pros: pusher feel and the start, stop and reset sequence expose a cheap module within seconds.
  • Pros: a 7750-based movement rocks the rotor and can stutter on chronograph start; a timegrapher reading catches low beat rate and weak amplitude.
  • Cons: you need the seller to open the back or send a movement photo, and not all will.
  • Cons: a decorated Asian clone photographs well, and opening the watch can end whatever return window exists.

Criterion 4: caseback engraving and reference stamps

  • Pros: engraving depth, font weight and the stamped reference are directly comparable against catalog images.
  • Pros: shallow laser etching reads flat and slightly grey next to a struck or deep engraving.
  • Cons: engraving is copied from published photos and is often close.
  • Cons: sellers recycle genuine photos, so the caseback in the listing may not be the watch you receive.

Criterion 5: crystal, hands and finishing

  • Pros: feuille hands with a diamond-polished bevel shift from black to bright as the watch turns, and that is difficult to fake.
  • Pros: sapphire with coating on both faces has a faint blue or purple cast at an angle that mineral glass rarely matches.
  • Cons: coating is invisible in most listing photos and a coated mineral glass looks close in a still.
  • Cons: hand finish is the last thing a low grade corrects, so it tells you about grade, not about authenticity alone.

Criterion 6: strap, buckle and packaging

  • Pros: buckle engraving and strap stitching are easy to compare at high magnification.
  • Pros: embossed calf with a printed grain looks different from real alligator scale structure.
  • Cons: straps get swapped between photos, so strap quality says little about the watch.
  • Cons: boxes and papers are sold on their own and prove nothing about the watch itself.

Criterion 7: legal and after-sale reality in the US

  • Pros: the entry cost sits far below a genuine Portugieser Chronograph, and the layout and look are there on the wrist.
  • Cons: counterfeit imports are subject to seizure at the US border, and the shipment is lost along with the money.
  • Cons: brand service centers will not touch it, independent watchmakers may refuse it, and there is no warranty and no resale value.

Use-case recommendation

If you want the Portugieser layout as a daily-wear chronograph and you accept no service, no resale and border risk, work from the checklist below and buy only from a seller who will open the caseback on camera and state the movement family in writing. If you want a watch you can service, insure and pass on, this reference is the wrong one to chase in replica form: a two-register chronograph from a brand with an authorized service network does the same job without the paperwork problem.

  1. Ask for the exact reference and generation, then confirm the movement family that belongs to it.
  2. Request an unedited photo of the caseback and one of the dial at an angle.
  3. Ask for a movement photo with the back removed, or a timegrapher reading.
  4. Compare sub-dial spacing and numeral cuts against catalog images.
  5. Confirm the return window in writing before paying.
  6. Check what your country's customs rules say about counterfeit imports before you order.