Short answer
A fake IW371436 usually fails on three things: the movement, the dial printing, and the caseback engraving. If you can see the back of the watch or hear it run, you can settle the question in under a minute. If you only have photos, start with the subdial layout and the weight of the fonts.
What a genuine IW371436 actually is
Ref. 371436 sits inside the IWC Portugieser Chronograph line, the 40.9 mm stainless steel bi-compax model that IWC built around a heavily modified Valjoux 7750 it calls calibre 79350. Two subdials, running seconds at 6 o'clock and chronograph minutes at 12, no date window, feuille hands, applied numerals, sapphire crystal with anti-reflective coating on both faces. IWC retired the 3714 family when it moved the Portugieser Chronograph to a 41 mm case and the in-house calibre 69355, which matters here: the replica trade built most of its tooling around this exact case shape, so fakes of the 3714 generation are common and often better than you would expect.
Start with the movement
Calibre 79350 beats at 28,800 vibrations per hour and winds automatically. Because it is a 7750 underneath, the chronograph has a feel you can learn in one afternoon. The pushers are firm, the reset snaps back hard, and the elapsed minutes hand moves in small jumps rather than a smooth sweep. Replicas usually carry a Chinese 7750 copy, a hand-wound Seagull, or in cheap cases a quartz module. A quartz chronograph whose 12 o'clock counter ticks once a minute instead of stepping in small increments is an instant no. When you get a movement photo, look at the rotor: fakes frequently misspell or misweight the engraving, get the jewel count wrong, or print the Probus Scafusia medallion in the wrong proportion.
Dial and hands
Genuine IWC dial printing is crisp under a loupe, with clean serifs and even spacing on the International Watch Co. and Schaffhausen lines. Replicas tend to print thicker or slightly blurry, and the IWC mark is often a touch too tall or spaced too wide. Subdial rings on fakes are sometimes printed flat where the real thing is stamped or applied. On the hands, the leaf shape should be slim with a sharp taper and reach close to the minute track. Short or stubby hands are a tell.
Case and caseback
The real case measures 40.9 mm across and about 12.3 mm thick. Many replicas run a half millimetre to a full millimetre thicker, which you notice once it is on the wrist. Caseback engraving is the fastest check with photos: genuine stamping is deep and even, while laser-etched fakes look shallow and grey under angled light. Serial numbers are worth searching. Genuine 3714 watches carried unique seven-digit serials, and fakes recycle the same blocks, so a quick search often turns up the identical number on several listings.
Price and paperwork
Clean secondhand examples of this reference generally trade in the low to mid four figures depending on condition, box, and papers. Nobody sells a genuine one for a tenth of that. A full IWC box and booklet with a three-figure price tag is a costume, not a bargain.
Quick checklist
- No date window at 3 o'clock
- Subdial hands reach the printed rings
- Automatic movement, 28,800 vph, chrono minutes at 12
- Crown and pushers feel firm, not mushy
- Caseback engraving deep and crisp
- Serial number unique across listings
- Asking price inside the normal market range
Before you pay
Ask the seller for a photo of the movement, a short video of the chronograph starting, stopping, and resetting, and the serial number. Pay with a method that gives you recourse. If the seller will not let a watchmaker open the back before money changes hands, treat that as your answer and walk.