Short answer
A watch sold as an IW371425 has to match the IWC Portuguese Chronograph baseline: 40.9 mm steel case, about 12.3 mm thick, two subdials, no date, Calibre 79350 inside. Fakes miss at least one of those numbers. Measure the thickness, count the subdials, and read the caseback before you look at anything else.
What the reference number covers
IWC catalogs the Portuguese Chronograph as reference 3714. The two digits after the dash code the dial color, case metal, and strap. Published lists show variants such as IW371417, IW371446 and IW371480. I could not confirm a published IW371425 entry. Ask the seller for a caseback photo and a watch card photo. A reference that appears in no catalog is a reason to stop and check again.
Numbers to compare
- Case diameter: 40.9 mm. Many fakes run 41.5 mm to 42 mm.
- Case thickness: about 12.3 mm. A fake built on an Asian 7750 lands at 13.5 mm to 14.5 mm.
- Subdials: 30-minute counter at 12, running seconds at 6. That is the Valjoux 7750 layout.
- Date: none. A date window at 3 is a defect.
- Water resistance: 30 m, marked on the caseback, not the dial.
- Crystal: domed sapphire standing above the bezel line.
Dial details
The real dial carries applied leaf hands and applied Arabic numerals, with the subdials in place of the 12 and the 6. Print quality is the tell. The IWC and SCHAFFHAUSEN line sits in a fixed relation to the 12 subdial ring. Fake printing is heavier and the subdial rings are wider. Compare the gap between the numerals and the chapter ring. The real dial has open space. Fake dials look crowded because every printed element is larger.
Caseback
The 3714 has a solid caseback. It carries engraved text: reference number, serial number, model name, water resistance. Real engraving is cut with sharp edges and even depth. Fake engraving is shallow and the font runs wide. The six-digit serial sits below the reference line.
Movement and pushers
Calibre 79350 runs at 28,800 vph with a 44-hour power reserve, based on the Valjoux 7750. It uses a cam and lever chronograph, so the start is firm and the reset snaps back. On a fake the top pusher feels soft and the reset can stick. Start the chronograph and watch the 30-minute counter at 12. It jumps one minute at a time. It does not sweep.
Price context
Steel 3714 examples trade on the secondary market at about $5,000 to $8,000 USD. Replica sellers ask a few hundred dollars. A steel Portuguese Chronograph offered under $2,000 is a replica at a replica price. Ask for the service history. A 79350 service from IWC costs several hundred dollars, and the paperwork is evidence a fake cannot produce.
What photos cannot show
The 3714 caseback hides the movement, so a photo set proves less than it seems. Crown feel, strap quality, and buckle stamping need the watch in hand. If you cannot hold it, insist on a caseback macro shot, a pusher video, and a written return window.