The Short Answer
A fake IW371633 usually gives itself away in three places before you ever open the caseback: thickness, chronograph feel, and the crystal. If the watch measures closer to 15 mm than 13 mm, if the pushers feel heavy and vague, or if the dial washes out under light the way single-coated glass does, you are not holding a genuine IWC Portugieser Chronograph. Printing, straps, and box contents matter, but they come second.
What IW371633 Actually Is
The reference belongs to the 41 mm Portugieser Chronograph line, the family IWC moved to its in-house calibre 69355. IWC reuses reference numbers across dial colors and case metals, so confirm the exact variant against IWC's own product page before you compare anything. A silver-dial steel model and a gold model share a family look but not a spec sheet.
Case and Proportions
- Genuine: about 41 mm across, roughly 13 mm thick, with polished bevels running into brushed flanks.
- Common tells: noticeable extra thickness, a caseback that sits too proud of the wrist, chunky lug undersides, and a crown that feels small or slick.
- Finishing: on the real thing the transition between brushed and polished surfaces is a hard, clean line. On copies it is often soft or wavy under a loupe.
Dial and Printing
Look at the subdials first. On the genuine watch the two registers are spaced with even margins, and the printed track around each one is thin and unbroken. Copies tend to crowd the registers, thicken the track, or print the minute hash marks with uneven gaps.
Then check the branding. The IWC SCHAFFHAUSEN lettering has crisp serifs and consistent stroke weight. Fakes often show slightly fat letters, blurry edges where the pad printing bled, or an AUTOMATIC line that sits a hair too low. The date window is another giveaway. Genuine apertures have a beveled, sharp-cut edge and the numeral sits centered with a matching font. Copies show a flat cut, off-center numerals, or a date wheel that does not line up with the frame.
Movement and Caseback
Calibre 69355 is a column-wheel chronograph with roughly 46 hours of reserve. Through the sapphire back you should see Geneva stripes on the bridges, blued screws, and a signed rotor. Copies generally run a modified Valjoux 7750 or a Seagull chronograph, so the decoration looks stamped rather than finished, and the rotor may spin with a rattle when you tilt the watch.
Pushers, Crown, and Feel
A column wheel gives a light, precise click when you start the chronograph. The 7750-based copies feel heavier, and the chronograph seconds hand often stutters on the first push or jumps when it resets. Wind the crown and compare. The genuine movement feels smooth with light resistance. A gritty or springy wind points to a cheap keyless works.
Crystal, Lume, and Strap
IWC fits sapphire with anti-reflective coating on both faces, which is why the dial stays legible at steep angles. Many replicas coat only the outside, so the dial goes milky or mirrors the room when you tilt it. Lume on the hands and markers should glow evenly and fade at the same rate. Straps are easier to swap than cases, so treat an aftermarket alligator strap as neutral evidence, but check the buckle engraving depth and the stitching density.
Franken Builds and Service Records
Some fakes use a genuine movement inside a copied case, which makes the movement check useless on its own. Ask for service paperwork with a matching serial, or send the watch to IWC for a quote. The brand will not service a counterfeit, and that answer alone settles the question.
Checklist Before You Buy
- Measure the thickness yourself with calipers.
- Tilt the watch toward a window and watch for dial haze.
- Start, stop, and reset the chronograph twice. Listen for a jolt.
- Compare dial printing at 10x magnification against IWC's official photos.
- Check the date font and window bevel.
- Verify the reference number on the caseback matches the dial variant.
None of these steps needs a watchmaker. They need ten minutes, decent light, and a willingness to walk away.