The most reliable single check on a watch offered as an IWC Portugieser Chronograph IW371655 is the caseback, specifically how the reference number is engraved into the metal, because that is the one detail a counterfeiter cannot lift from a product photo. Judge any candidate against five criteria, in this order: caseback engraving, dial printing and subdial layout, the movement seen through the display back, the case and pusher feel, and the paperwork. No single test proves authenticity by itself. Two or three matching checks raise confidence, and a watchmaker with the case open settles the question.

Caseback engraving: the top pick for a first pass

Reference suffixes in the Portugieser Chronograph family denote case metal and dial combinations, so confirm the exact IW371655 configuration against IWC published material before you compare anything else. Once you know what the back should read, study the cut. Genuine engraving is machined into the metal and shifts in brightness as you tilt the watch under a light. Etched or printed text on a fake stays flat, and the letter shapes are inconsistent between two examples of the same replica batch.

• Pro: needs only a loupe or a phone macro shot.
• Pro: case stays closed, so you can check before committing to a purchase.
• Con: polishing and service work can soften genuine engraving.
• Con: a good clone can copy the wording and still get the depth wrong.

Best for: remote buyers working from seller photos, where you cannot handle the watch.

Dial printing, subdials and hands

The classic Portugieser Chronograph dial places the chronograph minutes at 12 and running seconds at 6, with a clean face and no date aperture. Any candidate with a different subdial arrangement, an extra window or misaligned counters needs an explanation before you go further. Check the printed text at 3 and 9 for sharp edges and even spacing, the applied numerals for polish on the bevels, and the hands for a consistent brushed or polished finish front and back.

• Pro: high-resolution photos expose fuzzy printing and thick, blobby font.
• Pro: subdial spacing is a geometry problem that cheap copies fail.
• Con: lighting and camera angle create false alarms on genuine dials.
• Con: aftermarket dials on a genuine case confuse the picture.

Best for: buyers comparing two watches at similar prices who need a tiebreaker.

Movement and display back

A sapphire display back lets you see the chronograph movement, and finishing is where replica factories cut costs. Look at the rotor engraving, the perlage or circular graining on the plates, the polish on screw heads and the quality of the jewel settings. Pusher action belongs here too: a genuine column wheel or well-built cam chronograph starts and resets with a defined click, while a cheap module feels mushy or grinds.

• Pro: finishing quality is expensive to replicate at scale.
• Pro: pusher feel is testable in seconds at a meeting.
• Con: a movement swap inside a real case can fool a visual check.
• Con: you cannot judge lubrication or timekeeping without a timing machine.

Best for: anyone buying in person who can wind, set and run the chronograph.

Case, crown, pushers and crystal

Run a finger along the lugs and the caseband. Genuine cases have even bevels, sharp lug edges and a crown that turns with slight resistance and no wobble. The sapphire should show almost no reflection thanks to anti-reflective coating, and the pushers should sit square to the case with an even gap. Weight in the hand matters as well, though gold and steel variants differ, so compare against the correct metal.

• Pro: tactile checks catch flaws that photos hide.
• Pro: crystal coating is easy to see under a lamp.
• Con: genuine watches get refinished, blurring the edges.
• Con: feel is subjective without a reference example in hand.

Best for: in-person handoffs and auction previews.

Paperwork and professional authentication

Ask for the original invoice, warranty card and service history, then verify the serial with IWC or an authorized dealer. A full set does not guarantee authenticity, since papers are copied, but a missing set on a modern reference deserves a discount and extra scrutiny. For anything above a casual price, pay a watchmaker or authentication service to open the case and inspect the movement against the reference specification.

• Pro: the only step that gives a definitive answer.
• Pro: documented history protects resale value later.
• Con: costs money and time, and the seller may refuse.
• Con: a refusal is itself useful information.

Best for: high-value purchases and any deal where the seller pressures you to move fast. If you are knowingly shopping the replica market, these same checks tell you which tier of copy you are looking at, since the caseback, dial and movement are the three places where quality shows first.