A fake IW371651 is worth your money only when three things line up: a 39 mm case that matches the original profile, a chronograph dial with the subdials in the correct positions, and a day-date aperture cut at the right height. The strongest version now on the market gets all three right and runs a decorated automatic chronograph movement, which is why it is the top pick for most buyers. Cheaper copies miss the case thickness by a millimeter or more, print the dial text too heavy, and hide a quartz movement behind a solid caseback. Everything below is organized around the checks that separate a wearable replica from a throwaway.

What IW371651 Refers To

IW371651 sits inside IWC's Portofino line, a family built around clean dials and dress-watch proportions. The relevant shape is the Portofino Chronograph 39: a stainless steel case of roughly 39 mm, a chronograph with two subdials, a day-date window at 3 o'clock, and an automatic caliber from IWC's 69000 series with a power reserve that runs about two days. On the 39 mm Portofino chronograph, the upper subdial carries the chronograph counter and the lower one carries the running seconds, with the day-date tucked beside the crown.

Dial color, strap, and bracelet change with the exact sub-reference, so the reliable sources for what IW371651 ships with are IWC's own reference listing plus dealer photos that show the full set. When a seller cannot tell you which dial and strap combination they are copying, that is your first warning sign.

Case Size and Profile: The First Filter

Pros

  • A true 39 mm diameter keeps the slim, dress-watch stance on the wrist.
  • Lugs that taper and curve like the original let a strap sit flush against the case.
  • Case height close to the genuine watch means it still slides under a shirt cuff.

Cons

  • Oversized copies in 41 mm and up read as a different watch from across the room.
  • Extra thickness pushes the crown off the wrist line and makes the case look slab-sided.
  • Sharp, unpolished lug edges catch on fabric and scratch a desk.

Measure before you buy. Ask the seller for the case diameter, the thickness, and the lug width. A listing that gives only the diameter is hiding something.

Dial, Subdials, and the Day-Date Window

The dial is where most replicas fall apart, because printing and applied parts are hard to fake at a low cost.

Pros

  • Applied indices with polished facets that pick up light the way the original does.
  • Subdials sized and spaced to match the reference, not shrunk to fit a generic movement.
  • Dial text with the right weight and the right spacing between the logo lines.
  • A day-date wheel whose font matches the rest of the printing.

Cons

  • Painted or flat-printed indices that look dull next to the genuine article.
  • Subdial hands that are too short or too long for the register they sit in.
  • Text so heavy it looks stamped rather than printed.
  • A date wheel with mismatched numerals or a misaligned day window.

Compare a clear, straight-on photo against IWC's own reference image. The position of the subdials and the height of the day-date cut are the two details most buyers spot first.

Movement: What Should Be Inside

A chronograph replica that cannot run its chronograph is a costume piece, regardless of how the dial looks.

Pros

  • An automatic chronograph movement of the 7750 family with a working start, stop, and reset.
  • Running seconds that sweep instead of tick once per second.
  • A rotor with decoration and a caseback that shows it.
  • A power reserve measured in roughly two days of normal wear.

Cons

  • Quartz movements with a dead or decorative chronograph hand.
  • Subdials that are printed on the dial rather than driven by gears.
  • Loud rotors and a pusher that feels gritty or loose at the top of its travel.
  • Closed casebacks used to hide an undecorated movement.

Ask for a short video that shows the chronograph starting, running, and resetting to zero. Anyone selling a genuine automatic copy can produce that clip in a minute.

Crystal, Caseback, and Finishing

Pros

  • Sapphire crystal with an anti-reflective coating that stays neutral under indoor light.
  • Caseback engraving with correct reference text and clean lettering.
  • Brushed flanks and polished bevels that meet on a crisp line.

Cons

  • Mineral glass that scratches within weeks and shows a blue or purple cast.
  • Caseback text with wrong spacing, wrong font, or a misread reference number.
  • Brushing that runs in the wrong direction or bleeds across a polished edge.

Which Version to Buy

Buyers fall into three groups, and the right tier depends on how you plan to wear the watch.

  • Entry copies. Fine for a costume or a one-time event. Expect a quartz movement, a thicker case, and soft finishing. Skip these if you want the chronograph to work.
  • Mid tier. The best balance for daily wear: correct 39 mm dimensions, an automatic chronograph, a dial that holds up to a close look. Weak points are usually the clasp and a soft anti-reflective coating.
  • Top tier. Closest to the original in profile, dial printing, and movement finishing, with a bracelet that feels solid in the hand. The trade-offs are cost, longer waits, and stock that changes week to week.

For most people reading this, the mid tier is the pick. It gets the case size, the subdial layout, and the day-date window right, which are the three things anyone notices from a normal distance. Step up only if you care about movement decoration and clasp quality, and stay away from anything priced so low that a working automatic chronograph is impossible to build at that number.