The Short Answer
A fake IW371770 gives itself away in five places: case thickness, dial printing, subdial function, crown and pusher shape, and the movement under the caseback. The reference sits inside the IWC Portuguese Chronograph family, so every counterfeit has to fake the same set of visible features. Measure the case, loupe the dial, test the chronograph, and photograph the movement before you pay. No paperwork and no movement photos means no sale.
What the IW371770 Should Be
IWC groups the Portuguese Chronograph under reference numbers in the IW3717xx block. The trailing digits record case metal, dial color, and strap or bracelet configuration, which is why one IW3717 does not look like the next. Pull the spec sheet for the exact reference in hand: case diameter and thickness, crystal type, caliber, dial layout, and date position. A counterfeiter working from product photos will match the case size and miss the thickness, or match the dial and miss the movement.
Prerequisites
- Calipers that read to 0.01 mm
- 10x jeweler's loupe
- Phone with macro mode and a steady light
- Access to a watchmaker who will open the caseback
- The manufacturer's published spec sheet for the exact reference
Step-by-Step Checks
- Photograph the caseback and write down the full reference number and serial number as engraved.
- Match the reference against the published spec sheet for case size, caliber, and dial configuration.
- Measure the case diameter and the case thickness with calipers, then compare both numbers to the published figures. Counterfeit cases run thick.
- Check the dial layout element by element: subdial positions, date aperture, numeral style, hand shape, and minute track.
- Loupe the dial print and compare letter spacing, stroke weight, and the finish of the applied markers.
- Set the time, then run the chronograph through start, stop, and reset. Both subdial hands must return to zero and stay there.
- Run the chronograph for thirty minutes and confirm the elapsed-minute counter advances one step at a time.
- Feel the crown and pushers. Threads should engage without grit, and each pusher should have a distinct, short travel.
- Inspect the caseback engraving under the loupe for depth, edge quality, and font consistency with the reference.
- Have a watchmaker remove the caseback and photograph the movement, the rotor engraving, and the balance area.
- Compare the movement photos to the caliber the reference should carry. The Portuguese Chronograph line has used a Valjoux 7750-based caliber and, in later versions, an in-house 69000-series caliber.
- Ask for the original invoice, service records, or a written authentication from a third party, and check the serial against those documents.
Red Flags
- Case thickness above the published figure
- Soft or uneven dial print, or markers that sit proud of the dial
- Subdial hands that fail to reset to zero
- Rotor engraving with shallow or blurred lettering
- Caseback fonts that differ from the reference in weight or spacing
- A price far below the market for the reference, with a story attached
- A seller who refuses movement photos or an inspection
Where Copies Slip
Pad printing at this scale needs a plate cut for one dial. Counterfeiters reuse a single plate across several dial colors, so letter spacing drifts and the brand and city lines lose stroke weight. Case finishing is the second gap. Genuine cases get polished bevels that catch light in a straight line, while copies show rounded edges and inconsistent brushing. Movement finishing is the third. A decorated counterfeit rotor can pass in a phone photo and fall apart under a loupe, where the perlage is uneven and the engraving has no depth.
Before You Pay
- Timing rate and amplitude from a timegrapher, photographed on the spot
- A written return window long enough for an independent inspection
- Caseback removed in front of you, or a video call with the watchmaker
- Serial number on the case matching the serial on the paperwork
- A second opinion from a watchmaker who services IWC models