The fastest way to catch a fake IW371758 is to inspect the movement through the caseback: if the calibre does not match what IWC fitted to that reference, nothing else on the watch matters. This guide grades a watch on five checks, movement architecture, case and crystal geometry, dial printing and hands, caseback engraving and serial, and provenance paperwork, and then explains what even a high tier replica gets wrong.

Know the reference before you judge the watch

IW371758 sits inside the Portugieser Chronograph family: a two register chronograph with a counter at 12, running seconds at 6, feuille hands, applied numerals, and a steel case in the 40 to 41 mm range. IWC has produced this design across several generations, so case thickness, movement family, and dial text changed between references. Compare a candidate against photos of the exact reference, never against a generic search result for IWC Portugieser.

Pros

• Reference specific comparison removes most obvious fakes in under a minute.
• Published reference data gives you a hard checklist for case size and dial layout.

Cons

• Dealer listings and image searches routinely label the wrong reference.
• Generational overlap means two genuine watches of the same model can differ in small ways.

Use case: run this step first, before you ask a seller for movement photos.

Movement and caseback inspection

This is the highest value check on any IWC with a display back. Recent Portugieser Chronograph production uses an in house chronograph movement from the 69000 family, and that is where replicas fall apart. A column wheel chronograph should start, stop, and reset with a light, even push. A cam actuated clone often feels stiff on start and grinds on reset, and the running seconds hand can stutter or jump when you reset the chronograph.

Pros

• Crisp pusher action and a smooth sweeping seconds hand at 4 Hz match the documented calibre behaviour.
• Proper finishing shows Geneva stripes, perlage on the plates, and a rotor engraved with the brand motto.
• A movement number that matches the paperwork confirms the watch was assembled as one unit.

Cons

• Some references have solid casebacks, so you cannot see the movement at all.
• Decorated clones can look convincing in a photo and only fail in the hand.

Use case: if you can hold the watch, wind it, run the chronograph, and listen to the rotor before you commit. If you cannot, treat the movement as unverified.

Case, crown, pushers, and crystal

Case work is where mid tier fakes show themselves. Genuine Portugieser cases have crisp transitions between brushed flanks and polished bevels, lug tips that are sharp rather than rounded off, and a signed crown. Pushers sit flush and travel in a straight line. The sapphire crystal carries an anti reflective coating that shows only a faint tint under a light source.

Pros

• Consistent brushing direction and clean bevel lines suggest a properly finished case.
• A coated sapphire crystal keeps the dial almost black at an angle instead of washed out grey.

Cons

• Over polished edges and soft lug tips are the most common giveaways.
• Mineral glass with heavy reflections, or a crown with a smeared logo, points to a low tier build.

Use case: tilt the watch under a lamp. If the dial turns milky, keep looking.

Dial, hands, and printing

Applied numerals should sit proud of the dial with polished facets, and the brand signature should be thin, sharp, and positioned per the reference. Hands are mirror finished on the top facet and matte on the sides. Subdial rings should catch light cleanly, and any date window must be centred with the correct font.

Pros

• Applied rather than printed numerals, with even spacing around the dial.
• A subdial hand that reaches the printed track exactly.

Cons

• Fat, blurry text and flat printed numerals instead of applied markers.
• Subdial hands cut short, or a small seconds hand that sits off centre.

Use case: photograph the dial straight on and compare it side by side with reference images at the same size.

Serial, paperwork, and verification

Serial and reference numbers on the caseback should be engraved into the metal, not laser printed on the surface. The serial on the caseback, the movement, and the warranty paperwork should agree. An authorized IWC service point can confirm a serial and inspect the watch, which is the only check that settles the question. Note the limit of this test: counterfeiters reuse real serials, so a serial that checks out does not prove the watch is genuine.

Pros

• Matching serials across case, movement, and papers point to a complete, untouched watch.
• Service receipts and an original box raise confidence in a second hand purchase.

Cons

• Cloned paperwork is easy to produce and hard to distinguish in a photo.
• A verified serial alone proves nothing about the case in front of you.

Use case: on any private sale, budget a service inspection into the price before you buy.

What a high tier replica still gets wrong

Top tier fakes copy case dimensions, dial print, and the overall silhouette closely. They still miss movement finishing, the exact feel of the chronograph pushers, and long term rate stability. Winding action on a decorated clone is gritty next to a genuine in house calibre, and the rotor is louder. Buyers who accept a replica should also accept that IWC will not service it, that no factory warranty applies, and that resale value is close to nothing.

Bottom line

Authenticate the movement first, then the caseback engraving, then the dial, then the paperwork. If any single check fails without a clear explanation, treat the watch as a fake IW371758 and walk away, or pay a price that reflects replica status rather than genuine value.