Quick answer
Most fake IW371745 watches fall into two groups. The first is a copied case with a stock Valjoux 7750 inside. The second is a higher grade copy that matches the dial and caseback but still runs a third party movement. Both fail at the same point. Turn the watch over, look through the sapphire back, and read the movement. If the caliber number, jewel count, and rotor text do not match IWC's published spec for the reference, the watch is not genuine.
What the reference number covers
IW371745 sits in the Portugieser Chronograph family. IWC publishes case diameter, movement, power reserve, water resistance, and dial options for each reference. Public detail on this specific number is thin. Confirm the exact spec sheet with IWC or an authorized dealer before you judge a single watch.
Movement is the first tell
The Valjoux 7750 runs at 28,800 vibrations per hour. It uses a cam and lever system. On the wrist the rotor spins with a visible wobble. The chronograph pushers need a firm push and travel a long distance. An IWC in-house chronograph caliber in this line uses a column wheel. Pusher travel is short. Reset is crisp. A 7750 based fake also shows a small seconds hand at 9 o'clock and a 12 hour counter at 6. The genuine layout puts small seconds at 6 and the 30 minute counter at 12. That mismatch shows in photos alone.
Dial, hands, date
Look at the printed minute track under a loupe. Genuine print has sharp edges and even thickness. Copies show ink spread, uneven numerals, and subdial rings that sit off center. Check the date aperture. The frame should sit flush with the dial and the numeral font should match the rest of the printing. Copies use a generic font and a deeper cut.
Case, crystal, caseback
Caseback engraving on genuine IWC is cut with sharp borders. Reference number, case number, and material code sit in a fixed layout. Fakes use shallow stamps with heavier strokes. Check the crystal edge under a lamp. Genuine sapphire shows a faint color shift at the rim. Mineral glass on a cheap copy shows scratches after light wear.
Checklist before you buy
- Ask for a photo of the caseback and the movement.
- Compare the reference number and case number against IWC records.
- Check subdial positions. Small seconds at 6, counter at 12.
- Time the pusher travel. Short and light points to a column wheel.
- Request service history, box, and papers.
- Meet at a watchmaker for a movement inspection.
Limits of any guide
Photos hide print quality and pusher feel. A phone camera cannot show column wheel geometry. For a high grade copy, a watchmaker with the movement in hand is the single reliable check. IWC service centers and independent watchmakers can verify a serial number and service a genuine watch. Neither will authenticate a fake.