Answer first
A fake IW371631 is a counterfeit copy of an IWC Portugieser Chronograph. The genuine watch runs IWC's in-house calibre 69355, measures 41 mm wide and 13.1 mm thick, and lists near $8,000 in stainless steel on a leather strap. Copies break down in three places: case thickness, movement finish under the display back, and dial print. IWC's public catalog does not publish a full spec sheet for every IW3716 variant, so check the caseback number against IWC service records before you trust a listing.
What reference IW371631 is
The IW3716xx block covers the modern Portugieser Chronograph. That family uses a 41 mm case, two subdials at 12 and 6 o'clock, a date at 3, and a sapphire caseback. IW371631 falls inside that block. Treat any claim about its exact dial color, case metal, or limited run size as unverified until IWC confirms it in writing.
Check 1: case size and thickness
Genuine: 41 mm diameter, 13.1 mm thick, 20 mm lug width. Copies: 13.5 mm to 15 mm thick. The extra depth comes from a Valjoux 7750 base or a Chinese chronograph module stacked under a decorative plate. Measure with calipers. Ask for a side profile photo next to a ruler if you cannot handle the watch.
Check 2: movement under the display back
Calibre 69355 is an automatic column wheel chronograph with 27 jewels, 4 Hz beat rate, and a 46 hour power reserve. Look for the column wheel, the signed rotor, and Geneva stripes on the bridges. Copies use a stock 7750 or a Seagull movement with a stamped cover plate. Start the chronograph, let it run 30 seconds, then reset. A real 69355 snaps to zero in one motion. A 7750 based fake lags or jumps.
Check 3: dial, hands, and print
Genuine appliques are solid metal set into drilled holes. Copy markers sit on top of the dial or carry thin plating that flakes at the edges. Check the "IWC" and "International Watch Co." script under 10x magnification. Real print has sharp borders and even spacing. The date disc on the real watch centers in the window; copies show a gap or an off center numeral. Portugieser hands are leaf shaped, faceted, and polished on all surfaces.
Check 4: caseback, serial, and papers
The back carries the reference number, a serial number, and the Probus Scafusia stamp. IWC serials are sequential and can be verified through an authorized service center. A seller who refuses a serial check is selling a counterfeit. Boxes, cards, and manuals prove little. A warranty card with a serial that does not match the case is worthless.
Check 5: pushers, crown, and strap
Real pushers are rectangular, polished, and press with a firm click. The crown is fluted and signed with the IWC logo. Copies show soft edges, uneven engraving, and a crown that turns with grit. Straps on genuine watches are alligator with a signed pin buckle or folding clasp. Copies ship with embossed leather and an unsigned buckle.
Price test
A steel Portugieser Chronograph lists near $8,000 new from an authorized dealer in the US. Pre owned examples trade between $5,000 and $7,000 depending on condition and papers. A $400 "IW371631" on a marketplace site is a counterfeit. Current references do not sell at 5 percent of list through any legitimate channel.
Steps before you buy
- Ask for a caseback photo showing the full serial.
- Ask for thickness measured with calipers, or a side profile photo.
- Request a video of the chronograph start, stop, and reset.
- Call an IWC service center with the serial number.
- Walk away if the seller refuses any of the four steps above.