Short answer
A fake IW371491 rarely looks wrong in a phone photo. It looks wrong in the hand. The IWC Portofino Chronograph is a restrained steel dress chronograph, so the flaws sit in the small details: case thickness, dial printing under magnification, the depth and font of the caseback engraving, and the way the chronograph starts, stops and resets. If the watch in front of you is quartz-driven, half a millimeter or more too thick, or the chronograph hand drifts back to zero instead of snapping there, it is not genuine.
What the genuine reference should be
The IW371491 is a Portofino Chronograph from IWC Schaffhausen, a Swiss maker based in Schaffhausen. The Portofino family sits at the dressier end of the catalog: a slim steel case, a clean dial with two chronograph counters and running seconds, a date window, and an automatic chronograph caliber from IWC's 79xxx family, which is built on the Valjoux 7750 architecture. Because IWC has released several Portofino Chronograph references with different dial finishes, strap options and case metals, confirm the exact published spec for IW371491 on IWC's own product page before you inspect anything. Replica makers often copy the wrong spec sheet.
Before you start
- A 10x jeweler's loupe
- Digital calipers that read to 0.01 mm
- A timing machine, or a watch-rate app as a fallback
- Bright, angled daylight or a diffused lamp
- The official IWC spec page for the reference open on your phone
- Box, papers and service records if the seller has them
How to authenticate an IW371491
- Pull the factory spec for IW371491 and write down case diameter, case thickness, water resistance, crystal type and caliber number.
- Measure the case across the bezel at the 3 and 9 o'clock line with the calipers and record the number.
- Measure total thickness from the center of the caseback to the top of the crystal.
- Compare both measurements against the factory figures. Replicas usually run thick because of the movement spacer and a taller caseback. A 1 mm gap is a fail.
- Set the loupe on the dial at 12 and read the IWC wordmark. Genuine printing has sharp edges, even stroke weight and open counters inside the letters. Pad-printed fake dials look heavy and slightly greasy.
- Read the Schaffhausen text and the Swiss Made mark at the same magnification and check that no ink has bled onto the dial surface.
- Tilt the watch under the lamp and inspect every applied index. Each should sit flat, be polished on all faces and throw one straight reflection. Look for glue haze around the feet and a crooked marker at 12.
- Watch the small seconds subdial. A genuine 7750-based caliber beats at 28,800 vibrations per hour, so that hand moves in eight small steps per second. One jump per second means quartz.
- Press the top pusher to start the chronograph, the top pusher again to stop it, then the bottom pusher to reset. The stop should feel crisp and the reset should return the central hand to zero in one motion.
- Run the reset test three times and watch where the hand lands. A reset that drifts back over half a second, or parks off the zero mark, points to a low-grade movement.
- Unscrew the crown and turn it through a full wind. Threads should engage with light, even resistance. Gritty or stiff threads are a common replica fault.
- Set the date and watch the change at midnight. It should flip in one step near 12, not creep forward across an hour.
- Study the caseback engraving with the loupe. Factory engraving is deep with crisp shoulders. Laser-etched fakes look shallow, grey and soft at the edges.
- Read the reference number and serial number and match them to the warranty card and any service paperwork. A serial that does not match, or a reference stamp in the wrong typeface, ends the inspection.
- Check the crystal in daylight. Sapphire with the factory anti-reflective coating gives a faint blue-purple reflection and stays nearly invisible when you view the dial from a sharp angle.
- Time the watch on a timing machine. A healthy IWC 7750-based chronograph caliber should show 28,800 vph, amplitude above 250 degrees and a rate inside roughly plus or minus 10 seconds a day.
- Confirm the caseback style against the reference spec. If the genuine watch for that reference has a solid back and the watch in your hand has a display back, the case has been swapped.
- Take the watch to an IWC authorized service center or an independent watchmaker who services the brand. Opening the caseback and comparing movement finishing settles the question.
Red flags that need no tools
- A price far below the current secondary market range for the reference
- A seller who will not send macro photos of the caseback, clasp and dial printing
- Box and papers that look photocopied, or a card with a mismatched serial
- Chronograph pushers that feel mushy or make no audible click
- A rotor that rattles when you tilt the watch gently next to your ear
- Strap hardware with soft edges or a buckle that does not match the catalog photo
If you are deliberately shopping for a replica
The replica market for this reference splits into tiers, and the tier is decided by the movement. An Asian 7750 clone, a Swiss 7750 and a quartz module look similar on the outside and behave nothing alike on the wrist. Ask the seller which movement is fitted and for proof. Request macro photos of the dial printing, the caseback engraving and the clasp, plus a short video of the chronograph running and resetting. Confirm the case diameter and thickness in millimeters rather than trusting a listing photo. If you plan to wear the watch often, check that the crystal is sapphire with anti-reflective coating and that the pushers are screw-down or properly sealed, because this is where cheap builds fail first.
Verification that ends the argument
No photo checklist beats a bench inspection. An authorized IWC service center can open the case, verify the caliber, confirm the reference against the serial, and quote any service the watch needs. If a seller refuses that step, or wants payment before an inspection, walk away. On this reference the movement finishing, the caseback engraving and the factory dial printing are the three things a replica cannot match, and all three are visible once the watch is off your wrist and under a loupe.