Quick answer
A fake IW371633 almost always exposes itself in four places: the chronograph subdial spacing, the printed text on the dial, the depth of the caseback engraving, and the movement you can see through the sapphire back. The genuine IWC Portugieser Chronograph uses a 41 mm case and an in-house 69355 caliber, so any example with a third-party chronograph module, a tilted date wheel, or shallow laser etching is a replica.
What the genuine reference looks like
The IW371633 sits in the Portugieser Chronograph family: a 41 mm stainless steel case, roughly 13 mm thick, a domed sapphire crystal with antireflective coating on both faces, and two vertical subdials at 12 and 6 o'clock. The movement is IWC's own 69355 automatic chronograph, visible through a sapphire caseback, with a column wheel and a 46 hour power reserve. Those specifications are fixed. A seller who quotes different numbers is describing a different watch.
Before you inspect
- A 10x jeweler's loupe
- A bright, diffused light source, not a phone flash
- Digital calipers
- Reference photos of a known genuine example
- The seller's asking price and paperwork
Step-by-step authentication
- Place the watch under diffused light and study the dial text with the loupe. Genuine printing is razor sharp, evenly spaced, and sits flat against the dial surface.
- Measure the case across the 9 and 3 o'clock sides with calipers. It should read 41 mm, and the lug width should be 20 mm.
- Check the subdial positions at 12 and 6 o'clock. They should align perfectly with the vertical center line of the dial, with no drift toward one side.
- Set the date to the 31st and then to the 1st, and watch how the wheel jumps. A genuine 69355 snaps cleanly and centers the numeral in the aperture.
- Start the chronograph and reset it several times. The pushers should feel firm with a short, crisp travel, and the seconds hand should return to exactly 12 o'clock every time.
- Turn the watch over and read the caseback engraving with the loupe. Genuine engraving has visible depth and slightly beveled edges, while replicas often show a flat, sandy texture.
- Compare the reference number and serial against the IWC product archive for the exact case and dial combination.
- Look through the sapphire back at the rotor and bridges. Replicas typically hide a stock ETA or Seagull movement under an engraved rotor plate.
- Weigh the watch in your hand and check the bracelet or strap hardware. Genuine clasps close with a solid, damped click and show crisp stamping.
- Ask for the warranty card, box, and service history, then match the card number to the caseback serial. Mismatched paperwork is a hard stop.
Common red flags
Subdials that sit too close together or too far apart, a crown that wobbles when you wind it, painted rather than applied indices, a caseback that simply says Swiss Made with no reference or serial, and a price far below the market for a steel Portugieser Chronograph. Any single flag is worth a second look. Two flags together mean you should walk away.
Getting a definitive answer
Dial and case checks catch most replicas, but the movement is the final word. A qualified watchmaker can open the caseback, confirm the caliber number on the bridges, and check that the column wheel is present. For a purchase at this price level, that inspection is worth the fee, and it protects you if the seller refuses to allow it.