The short answer
The IW371491 is a stainless-steel IWC Portugieser Chronograph, so "fake IW371491" means a copy of that watch rather than a different reference. The genuine article runs a 40.9 mm case, about 12.3 mm thick, twin subdials at twelve and six, no date, sapphire front and back, and IWC calibre 79350, a modified Valjoux 7750 with a 44-hour reserve. Copies range from a quartz lookalike with a printed dial to a mechanical first copy that is genuinely well built. The tells below sort one from the other, and most of them cost nothing to check.
The number on the caseback is the first filter
I start here because it is the cheapest check and the one fakers fail most. Genuine IWC casebacks carry a deep engraved reference, a case number, a serial, the water resistance rating, and the Probus Scafusia line. The edges of that engraving are sharp under a loupe. Laser-etched copies look grey and flat, and the lettering often crowds or runs uneven on the curve.
Then search the serial. Real numbers are one per watch. If the same digits turn up in listings on three continents, someone copied a photo of a real back and reused it. On this reference the reference number itself should match the dial variant you are shown. A seller who posts a black dial against a reference that never shipped that way is either careless or selling a franken watch.
Movement tells
Calibre 79350 sits on the 7750 architecture, so behaviour gives it away. Wind it and you should feel the rotor's two-way wobble. The chronograph pushers have short, firm travel and a clear click. Reset is instant and the hands land dead on zero. If the chronograph seconds stutter, if the minute counter jumps, or if the hands park a hair off zero after reset, the movement is not what the listing claims.
Pop the back, or ask for a photo of it. The rotor should be signed, skeletonised, and marked with the calibre. Plenty of first copies carry a plain rotor with misspelled text or none at all. Also check the running seconds at six: a smooth sweep means mechanical, a one-tick-per-second jump means quartz, and no amount of case finishing hides that.
Dial, hands and glass
Applied numerals on a real Portugieser have polished bevels and crisp edges that catch light in a single line. Copies often show rounded or painted markers. The leaf hands should be thin at the base and taper cleanly. If yours are blued, blued steel is heat treated and reads deep blue, while painted blue looks flat.
Look at the crystal from a raking angle. Genuine IWC uses sapphire with anti-reflective coating on both faces, so the dial stays legible and reflections are faint. Cheap copies show a heavy purple or blue sheen on one side only, or a mineral glass with a visible green edge. Subdial hand length matters too. On a good watch the hands reach into the track and stop short of the centre by the same amount on both registers.
Case, crown, pushers, strap
Run a fingertip along the join between the brushed flank and the polished lug. Genuine cases have a hard, straight transition. Copies get tumbled in a drum and the edge feels soft and rounded. Check the thickness too. If the case measures 13.5 mm or more, the midcase was built around a different movement.
The strap is another giveaway. Original alligator has irregular scale patterns and a hand-finished edge; embossed calf looks stamped and repeats. Look at the clasp engraving as well, since it is shallow and machine-flat on most copies.
What a first copy gets you, and what it does not
Buying honestly matters here. A first copy is not an IWC. It will not be serviced by IWC or covered by their warranty, resale value sits near zero, and the stated water resistance is a guess unless a watchmaker pressure tests it.
In the United States, importing or selling counterfeit goods is illegal, and CBP seizes shipments. A seller who calls a copy genuine is committing fraud. If you buy a first copy, buy it as a copy, at a copy price, and keep the receipt that says so.
Pre-purchase checklist
- Ask for a clear photo of the caseback and search the serial before paying.
- Confirm the reference matches the dial, hands and strap in the photos.
- Ask for a movement photo showing the signed rotor and calibre mark.
- Request a video of the chronograph start, stop and reset.
- Check the running seconds for a sweep, not a tick.
- Measure or ask for case diameter and thickness in millimetres.
- Get the seller's return policy in writing, since most disputes are settled before shipping, not after.