For a fake Type XXI 3817, the top pick is the automatic build with a genuinely working flyback chronograph and a live 24-hour day/night subdial. Those two functions are the reason the Type XXI exists, and they are the two things you cannot add later. Dial print, lume tone, strap and caseback engraving can all be corrected or replaced after purchase. The criteria below sort replica versions by movement honesty first, then case profile, dial legibility, and how clearly the seller answers questions about the 24-hour hand.

What the Type XXI 3817 is, before you compare copies

Breguet's Type XX and Type XXI line descends from chronographs supplied to French naval aviation in the 1950s and 1960s. The modern Type XXI added a 24-hour indicator and a flyback function, which restarts the chronograph with one push instead of stop, reset, start. Reference numbering across the family is easy to confuse, and sellers often blur 3810 and 3817 language together. Check the official product page for the specific reference before you compare any replica, because case diameter, dial layout and movement details shift between generations. Bring these details into every listing you read: the fluted case band, the Breguet-style hands, the sapphire caseback, and the pusher shapes.

Criterion 1: Movement and the 24-hour subdial

This is the dividing line between tiers. A live 24-hour hand and a real flyback need a chronograph module that costs money, and that cost is the first thing cheaper builds cut.

  • Run the chronograph, then press the flyback pusher. The seconds hand should jump back to zero and keep running without a stop.
  • Watch the 24-hour subdial across an hour. A hand that never moves means the subdial is decorative.
  • Ask for a video of the caseback side with the rotor in motion. A closed back is not automatically bad, but on a Type XXI it hides the weakest part of the build.

Use case: if you plan to use the chronograph, only this tier is worth the money.

Criterion 2: Case profile, crown and pushers

A replica lives or dies on silhouette. The Type XXI has a comparatively slim mid-case, a fluted band, and pump-style pushers that sit close to the crown.

  • Good: correct lug width, a crown sized to the case, pushers aligned with the crown on one axis.
  • Weak: a case that is visibly thicker than the reference, an oversized crown, or pushers that sit proud of the band.
  • Ignore claimed water resistance. Nobody pressure tests a replica, and gaskets are the first thing to fail.

Criterion 3: Dial, hands and lume

Dial texture and print weight are where photos help most. Ask for a straight-on shot in natural light, not a studio render.

  • Good: crisp printed text, concentric subdial rings, hands with a clean sweep and no flashing left on the edges.
  • Weak: milky Arabic numerals, lume that glows green when the reference uses a warm off-white, or a chronograph seconds hand that rests off zero.
  • Check the date window shape and position against the reference you are copying, not against another replica listing.

Option 1: Automatic with working flyback and live 24-hour subdial

Pros: the functions match the reference, the dial usually gets the most attention, the case profile is closest to correct, and the watch can be worn daily. Cons: the highest price tier, thicker than the genuine watch, and it will eventually need a service that many local watchmakers will refuse. Use case: someone who wants one watch to wear and actually use the chronograph.

Option 2: Automatic with a frozen 24-hour subdial

Pros: cheaper than the flyback version, often the same case and dial as tier one, and it still winds and runs. Cons: one subdial is dead weight, flyback is usually missing, and that mismatch is visible to anyone who knows the reference. Use case: a collection piece or a display watch where the subdial is rarely noticed.

Option 3: Quartz or entry automatic builds

Pros: lowest cost, accurate timekeeping, easy to replace. Cons: a ticking seconds hand, wrong case thickness, simplified dial printing, and hands that rarely match the reference profile. Use case: costume, photography, or a first test of whether you like the design before spending on a better build.

How to vet a seller

  • Ask for movement photos and a running video, not product renders.
  • Ask directly which functions work. A vague answer about the 24-hour subdial is your answer.
  • Request the actual watch shown in the photos, with a detail you choose visible in frame.
  • Confirm the return window in writing, and pay with a method that has a dispute process.

Verdict

Buy the flyback-correct automatic if the budget allows, because the 24-hour subdial and flyback are the whole point of the Type XXI 3817. Step down to the frozen-subdial automatic only if price matters more than function, and treat quartz builds as props rather than watches.