The short answer

A fake IW371663 is a counterfeit of the IWC Portugieser Chronograph 41, a 41mm steel chronograph running IWC's in-house calibre 69355. Copies are common, and most of them fail in the same three places: the movement you can see through the display caseback, the printing on the dial, and the way light moves across the case flanks. Learn the genuine spec sheet and you can sort a serious replica from junk in about two minutes.

What the real IW371663 is

The reference sits inside the Portugieser Chronograph 41 family. IWC specs that line with a stainless steel case around 41mm wide and roughly 13mm thick, a domed sapphire crystal with anti-reflective coating on both faces, and water resistance of 3 bar. The dial is bicompax: chronograph minutes at 12, running seconds at 6, date at 3 on most versions. Inside is calibre 69355, an automatic chronograph with a column wheel, a 46-hour power reserve, and a 4Hz beat. The caseback is sapphire, which matters more than anything else here. The movement is supposed to be visible.

Dial color and hand finish shift between references in the family, so confirm the exact configuration of IW371663 against IWC's own listing before you judge anything else. Getting the colorway wrong is the first mistake people make, and it is an easy one to avoid.

The five tells that actually matter

• The movement. Real 69355 has a decorated rotor, Geneva stripes on the bridges, crisp engraving, and a column wheel you can sometimes glimpse at the edge. Replicas usually drop in a third-party chronograph movement, often a 7750 derivative, which explains why so many fakes exist in the first place, since the older Portugieser 3714 references used a modified 7750 base. Look for a wobbling rotor, uneven lettering, and a power reserve that dies overnight instead of running two days.
• Dial printing. The IWC wordmark and Schaffhausen sit on a strict baseline with even spacing. On fakes the text is often too bold, sits too low, or smears under a loupe. The subdial rings should be perfectly concentric.
• Date window. Genuine windows have a slight bevel and a matching date font. Copies show a raw punched hole and a font that belongs to a different watch.
• Case finishing. The transition from brushed to polished surfaces is sharp and consistent on the real thing. Fakes round it off or leave polish where brush should be.
• Strap and buckle. Engraving on a genuine pin buckle is deep and clean. Replica buckles look stamped, and the leather is usually corrected-grain rather than alligator.

What the pushers and crown tell you

Push the chronograph start pusher. A genuine column-wheel movement has a light, distinct click and the seconds hand jumps cleanly from zero. A cam-actuated fake feels mushy, and the hand often stutters or lands slightly past 12. Wind the crown and listen. Screwing it down should feel smooth with no grit. On cheap replicas the crown threads are cut badly and never seat with a firm stop.

The price math never lies

Secondary market Portugieser Chronographs trade in the thousands, and even well-worn examples hold real value. Any IW371663 offered for a few hundred dollars is a replica, full stop. If a seller claims it is genuine but cannot produce papers, a service history, or a serial you can verify with IWC, walk away. Replica sellers rarely post the reference number in their own listings, which is why searching the number often leads to identification threads rather than store pages.

If you are shopping for a replica on purpose

Be clear about what you are buying. There is no warranty, no service network that will touch it, and no resale value. Movement failures are the usual outcome within a year or two, and a watchmaker who works on ETA and Sellita will often refuse the job. Ask the seller for a real photo of the caseback and the movement, not a stock image. Ask what movement is inside and what the power reserve actually measures. Vague answers are your answer.

If you think you bought a fake

Take it to an independent watchmaker and have the caseback opened. That single step settles it. Then decide whether you want a refund, a chargeback, or a lesson. If you paid by card or a buyer-protected platform, file fast, since these windows close. Do not send the watch back before you have a written resolution, because return addresses for counterfeit sellers tend to go quiet.