Short answer
IW371643 is an IWC reference number in the Portugieser Chronograph family. The 3716 prefix covers a 41 mm steel chronograph with an in-house automatic movement. A search for "fake iw371643" means one of two things: a buyer looking at a counterfeit, or an owner checking a watch they already hold. Counterfeit copies of this reference exist. The genuine watch has a column wheel chronograph movement, an IWC signature on the dial and caseback, and a serial number IWC can verify.
The genuine reference
IWC sorts its catalogue by reference. Numbers under 3716 belong to the Portugieser Chronograph. The case measures 41 mm across and about 13 mm thick. Lug width is 20 mm. Water resistance is 3 bar. The dial carries a small seconds subdial at 6 o'clock and a chronograph minutes counter at 12 o'clock.
IWC built several dial and case variants under the 3716 prefix. I could not confirm which dial color and case metal IW371643 uses from public records. Check that detail with an IWC boutique or the IWC catalogue before you treat any listing description as correct.
Movement
The genuine watch runs IWC calibre 69355. It is an automatic chronograph with a column wheel, about 46 hours of power reserve, and a beat rate of 28,800 vibrations per hour. Counterfeits use a Seagull ST19, a Chinese copy of the Valjoux 7750, or a Seiko meca-quartz module. A quartz fake gives itself away at the small seconds subdial: the hand at 6 o'clock jumps once per second instead of sweeping.
Checks that catch most fakes
- Caseback engraving. Genuine text is crisp and shallow. Fake engraving looks deep, rough, or paint filled.
- Subdial spacing. Copies place the two subdials too close to the center.
- Date window. Fakes show a white plastic ring around the date disc.
- Pusher feel. Genuine pushers have short, firm travel. Copies feel mushy.
- Weight. A steel case with a column wheel movement feels dense. Plated brass does not.
- Serial and reference. Both are engraved between the lugs on the genuine watch.
- Papers. IWC cards carry a matching reference and serial number.
- Price. A new steel Portugieser Chronograph sells above 6,000 USD at retail in the US. A new offer far below that figure is not genuine.
Legal risk
18 U.S.C. 2320 makes trafficking in counterfeit goods a federal crime. A first offense carries up to 10 years in prison. A repeat offense carries up to 20 years. U.S. Customs and Border Protection seizes counterfeit watches at ports of entry and destroys them. IWC can also sue sellers under the Lanham Act for trademark infringement. Owners of counterfeits have no recourse. IWC and its service partners will not service, repair, or supply parts for a fake.
Verification
Take the watch to an IWC boutique or an authorized dealer. Staff can check the serial number against IWC records. A watchmaker can open the caseback and identify the movement. Independent authentication services also exist. Ask for the result in writing if you are buying.