Short answer
A fake IW371473 fails on three points: the subdial layout, the construction of the hands and numerals, and the depth of the caseback engraving. Check those three first. Photos on a listing page do not settle the question. A loupe and a chronograph timing app settle it in ten minutes.
The reference in question
IW371473 sits in the IWC Portugieser Chronograph family, the IW3714 line. The family runs a 40.9 mm steel case, a leaf hand dial with two subdials, and a date at 3 o'clock. Full spec sheets for every edition number are thin, so confirm the dial color and strap on the warranty card before you compare details. The checks below hold across the line.
Check 1: subdials at 12 and 6
Running seconds at 6 o'clock. Chronograph minutes at 12. Nothing at 9.
Most fakes start from a Valjoux 7750 clone or a Seagull ST19. Both movements put running seconds at 9 o'clock. Builders add a module to relocate the hand, and the module shows. Measure the gap from each subdial edge to the center pinion. On the real dial the gap is equal. On a converted movement it is not. Check the subdial hands too: leaf shape, blued or rhodium finish, hub cap sitting flush.
Check 2: dial print and hands
- Arabic numerals are applied, not printed. They sit above the dial surface and cast a thin shadow at a raking angle.
- The minute track is printed. On fakes the ticks thicken and blur under 10x magnification.
- Leaf hands taper to a point. Stamped copies show a flat tip and a visible seam.
- The date window has a sharp cut and the numeral sits centered. Off-center dates are a common fault on copies.
Check 3: crystal and case
Sapphire with anti-reflective coating on both faces on most editions. Under a desk lamp a real one shows a faint blue or purple tint at an angle and almost no glare head on. Fakes coat one side, or use mineral glass that a steel pin will scratch.
Case flanks are polished. The bezel is thin. Inspect the lug holes and the inner faces of the lugs for tool marks. Pull the crown through three positions; each one should stop with a clean click. Press each pusher. Start, stop, reset. No grind, no soft travel.
Check 4: caseback and movement
Engraving depth is the fastest tell. Laser etching on a fake reads grey and shallow under a loupe. Engraved text on a genuine case has walls you can feel with a fingernail.
Where the caseback is transparent, read the rotor text and study the perlage. Calibre 79350 is a modified Valjoux 7750 running at 28,800 vph with about 44 hours of reserve. Later references use the in-house 69355 at 46 hours. Fakes copy the rotor engraving and skip the bridge work. Sharp screw slots, uneven striping, and a rotor that wobbles when you turn the watch are all failures.
What a photo cannot show
Winding feel. Rotor noise. The snap of the chrono reset to zero. Weight in the hand. For a remote purchase, ask for a video with sound and a clip of the chrono resetting.
Bottom line
Three checks catch most fakes: subdial layout, hand and numeral construction, caseback engraving depth. If one fails, stop. A reference number printed on paperwork proves nothing by itself.