Short answer
The fake Type XXI 3817 is a replica of the Breguet Type XXI Transatlantique, reference 3817, a 42mm steel flyback chronograph. Replicas come in three rough tiers, and the honest split is this: the best ones copy the case, dial layout and bracelet closely enough to fool a glance. Almost none of them copy the movement, because the genuine uses Breguet calibre 584Q/2 with a true flyback function. Most replicas sit on an Asian 7750 or 7753 base with a 24-hour module. So the useful question is not whether it is a fake. It is which level of fake you are holding.
What the genuine 3817 is
You need a baseline before you can judge a copy.
- 42mm steel case, around 14mm thick, with Breguet's fluted coin-edge caseband
- Calibre 584Q/2, automatic, flyback chronograph, roughly 48 hours of power reserve
- Running seconds at 9, a 24-hour day and night subdial at 3, chronograph minutes at 6
- Date window tucked between 4 and 5, with a font specific to Breguet
- Central chronograph seconds hand, luminous baton hands, luminous Arabic numerals in the house typeface
- Sapphire on both sides, display caseback, reference engraving starting with 3817ST on steel models
The flyback test
This is the one check that settles most arguments. Start the chronograph, let it run for twenty seconds, then press the lower pusher. On a real flyback the hand snaps to zero and keeps running in one motion. On a standard Valjoux-style chronograph the lower pusher is locked out while the chrono runs, so nothing happens, or you feel a hard stop. Ask any seller for a video of exactly that sequence. If they send a clip of start and stop only, they are avoiding the question.
Dial and case details worth zooming in on
Breguet's printing is crisp and slightly raised in look. Cheap replicas print soft, and the letterforms in BREGUET drift wider or narrower than the original. Look at the caseband flutes next. Genuine flutes are machined, evenly spaced and sharp to the touch. Stamped flutes look mushy and often lose depth near the lugs. The subdial spacing matters too. On the 3817 the three registers sit in a specific triangle, and a movement with a different base often pushes them off center. Date font is a tell as well, since replica date wheels usually come from a generic parts bin. Finally, check lume color across the numerals, hands and pip. It should match.
Weight will not save you
A 42mm steel case with a hefty chronograph movement inside already feels substantial, so a good replica feels close to right on the wrist. Do not use weight as your main filter. Use finishing: how the brushed flanks meet the polished bevels, whether the bracelet end links sit flush, whether the crown screws down without grit. Plated brass cases and thin stainless steel show up here long before they show up on a scale.
How the tiers compare
- Budget. Stamped case, mineral glass, printed subdials, wrong register spacing, no flyback, rough bracelet.
- Mid. 316L steel, sapphire crystal, sharper dial printing, decorated rotor, correct thickness, still no flyback and a 24-hour subdial that simply trails the hour hand.
- Top 1:1. Closer case profile, better hand shapes and lume, engraving that resembles the original, movement bridges and rotor finished in the Breguet style. Still not a flyback in most builds.
Questions to ask before you pay
- Video of chrono start, stop and the flyback attempt
- Calipers photo showing case thickness
- Macro shots of the dial at 3, 6 and 9
- Which movement base is inside, and whether there is a 24-hour module
- Close-ups of the caseback engraving and the clasp markings
My read
The 3817 is a decent replica candidate because the dial is busy and the layout is unusual, which hides small errors from anyone standing a foot away. The fluted caseband is the hard part, and it is where most copies fall down under close inspection. If you want the look on your wrist, mid tier or better gets you there. If you want a working flyback chronograph, buy the real thing, because no replica at any price delivers that function properly.