Short answer
A fake IW371790 is a counterfeit that carries the IWC reference number IW371790 on the caseback. That reference is not among the most copied IWC chronographs, so a large share of watches sold under it are generic chronographs with a stamped number rather than close copies of a real model. Authentication comes down to four checks: whether the reference appears in IWC's own records, whether the case measures and weighs what the genuine watch does, whether the dial printing holds up under magnification, and whether the movement matches what the caseback claims.
What you need before you start
- A 10x jeweler's loupe
- Digital calipers that read to 0.01 mm
- A scale accurate to 0.1 g
- The spec sheet for the reference from IWC or an authorized dealer
- Photos of a known genuine example from an auction archive or a collector forum
How to check an IW371790 step by step
- Ask IWC customer service or an authorized dealer whether IW371790 exists as a catalog reference, and request the original case diameter, thickness, lug width, and movement caliber.
- Measure the case across the bezel at 3 and 9 o'clock with the calipers, then measure the thickness from caseback to crystal, and write both numbers down.
- Weigh the watch on its strap or bracelet and compare the result with a genuine example of the same reference and the same strap material.
- Read every line of dial text under the loupe: the brand name, the model name, the subdial markings, and the country of origin at 6 o'clock.
- Check the hands against a reference photo, comparing length, tip shape, and how the subdial hands line up with their scale markers at zero.
- Study the caseback engraving, the reference number, and the serial number, looking for shallow stamping, uneven letter spacing, or a serial that does not fit the model's production range.
- Open the caseback with a proper tool or have a watchmaker do it, then compare the movement finish, the rotor engraving, and the jewel count with the caliber IWC lists for that reference.
- Inspect the strap, buckle, deployant, and end links for sharp edges, loose tolerances, and hardware that does not match the case finish.
- Compare the box, warranty card, and booklet against known IWC packaging, and check that the serial on the card matches the serial on the caseback.
Red flags that point to a counterfeit
- The seller cannot name the movement caliber or the case diameter
- The reference number is stamped with a laser engraver instead of pressed or engraved into the metal
- Dial text sits low on the surface, looks flat, or shows fuzzy edges under 10x magnification
- The watch is sold as a factory second, an overrun, or a sample, with no papers
- The price sits far below the market range for that reference, even for a used example
- The serial number appears in online listings for a different model
What even a good replica gets wrong
Replica makers spend their budget on the parts a buyer sees first: the dial, the crystal, and the bracelet. The caseback engraving, the crown threads, the pusher travel, and the movement finish get less attention. Run the chronograph start, stop, and reset a few times and watch how the seconds hand snaps back. A genuine column wheel or cam mechanism returns to zero in one motion, while a modified base movement tends to overshoot or lag. That single test exposes most fakes faster than any visual check.
When to walk away
If the reference does not appear in IWC's records, stop there. A model number that no catalog lists is either a typo or an invention, and no amount of dial inspection will turn an invented number into a real watch. If the reference does exist but the measurements, weight, or movement do not match the spec sheet, treat the watch as a counterfeit and end the transaction.