The fastest way to answer the question "is this IW371631 fake?" is to verify two things before you look at anything else: the movement inside the case, and the reference number against IWC's own production records. An authorised IWC service centre, or a watchmaker who regularly handles IWC calibres, can open the watch and confirm both. Everything else, from dial printing to bracelet finishing, is supporting evidence. If the movement matches and the reference checks out, the watch is very likely genuine. If either fails, nothing else matters.
Why the reference number comes first
The 3716 family of the Portugieser Chronograph carries a long list of reference numbers, and the last digits usually encode case metal, dial colour or a market-specific variant. That makes the number itself a test. Before you judge the watch, confirm that IW371631 appears in IWC's catalogue or service system as a real production reference. A number that cannot be matched to any official record is a red flag on its own, and copy sellers frequently invent plausible-looking numbers because buyers rarely check them.
- Pros: It takes minutes, costs nothing, and a mismatch is decisive.
- Cons: Genuine references exist in small runs, so a dealer may need to look twice before confirming.
Best for: any private sale where you cannot inspect the movement before paying.
The movement and the caseback
The modern generation of the Portugieser Chronograph runs on IWC's own 69355 calibre, visible through a sapphire display back. Older 3714-generation watches used the 79350, a heavily reworked Valjoux 7750 base, and many of those have a closed caseback, which means the watch has to be opened to be judged. Replicas almost always put a decorated generic chronograph movement behind a display back and print IWC text over it. Decoration can be copied well. Architecture cannot: the plate layout, the position of the gear train and the shape of the bridges differ from a stock 7750 in ways a watchmaker recognises at a glance.
- Pros: The clearest single tell, and it is hard to disguise under a display back.
- Cons: You cannot do it yourself, and opening a watch carries a small risk if the person doing it is careless.
Best for: any watch sold with a display back and a story about a "rare" variant.
Dial, hands and subdial layout
The Portugieser Chronograph uses a vertical bicompax layout: a 30-minute counter at 12 o'clock and running seconds at 6 o'clock, with no third subdial. The hands are leaf-shaped feuille hands, the numerals are applied, and the minute track is printed as a fine rail. Copies usually fail on small things here. The subdial spacing drifts, the printing loses sharpness under magnification, the applied numerals sit too flat against the dial, and the hand finish looks brushed rather than polished on the top surfaces with angled edges.
- Pros: You can assess it in normal light without tools, and photos at an angle reveal a lot.
- Cons: Excellent copies exist, so a clean dial proves little on its own.
Best for: a first-pass filter when you are looking at listings online.
Case, crystal and finishing
The case should measure roughly 41 mm across on the current generation, with lugs that flow into the caseband rather than sitting on top of it. The sapphire crystal should be clear from every angle, with anti-reflective treatment that does not turn milky or purple. Run a fingernail along the case edges: genuine cases are polished and brushed in separate zones with a crisp, defined line between them. Copies often blur that transition or polish the whole case because it is cheaper to finish.
- Pros: Contact-based checks work even on a watch you cannot open.
- Cons: Refinishing and polishing can soften a genuine case, so a worn original can look worse than a new copy.
Best for: in-person viewings, where your hands can do what a photo cannot.
Papers, pricing and the seller's story
Documentation should match the reference, the serial and the watch in front of you. Ask for the original invoice, the warranty card or booklet, and service history. Then look at price in context: compare the asking figure with what clean examples of the same reference actually trade for on public marketplaces. A price that sits far below that range is not a bargain, it is a signal. So is a seller who cannot say where the watch came from, or who pushes you to decide quickly.
- Pros: Free, fast, and it filters most bad listings before you travel anywhere.
- Cons: Documents can be forged, so paperwork supports a verdict rather than replacing one.
Best for: online listings, before you commit to a viewing or a deposit.
A short authentication checklist
- Match IW371631 to an official IWC production record.
- Confirm the calibre inside, ideally by a watchmaker, not by the seller.
- Compare caseback engraving, serial and reference with the paperwork.
- Check subdial spacing, hand finish and dial printing under magnification.
- Inspect case finishing zones and crystal clarity in daylight.
- Compare the asking price with real market results for the same reference.
- Ask about service history and be wary of pressure to buy quickly.
One practical note: in many countries, buying or reselling a counterfeit watch is illegal, and a fake carries no manufacturer service, no parts support and no resale value. If a watch fails two or more of the checks above, walk away rather than negotiate.