The short answer: a fake Type XXI 3817 is a replica of Breguet's steel flyback chronograph, and the better ones get the case and dial close enough that nobody at arm's length will blink. What they cannot copy is movement behavior. If you care about the flyback, the 24-hour track, or anything past looks, that is where the money goes.

What the reference actually is

The Breguet Type XXI ref. 3817 is a 42mm steel pilot's chronograph, best known with a slate grey dial, applied numerals, and the hollow-tipped hands Breguet has used for decades. It carries a 24-hour indicator, a date window, and the retour en vol flyback function, which lets you restart the chronograph with one push of the lower pusher. The automatic calibre sits in the 584Q family with a power reserve near two days. The case middle is fluted, a Breguet signature that runs around the band like a coin edge.

Where replicas hold up

Case diameter, lug shape, and overall silhouette are rarely the problem now. Most 3817 reps land within a fraction of a millimeter and wear the same on a 7-inch wrist. Dial printing has improved a lot: the Breguet script reads clean under a loupe, the numerals sit at the right radius, and sapphire with anti-reflective coating is standard even on cheaper builds. Strap hardware is decent too. A rubber or leather strap with a signed buckle looks the part.

Where they give themselves away

Fluting depth is the first thing I check. On the genuine case the grooves are cut deep and even; on a rep they tend to be shallow and slightly rounded at the edges, and the polished flats between them catch light in a flatter way. Then the hands: genuine Breguet hands are thin, sharply finished, and hollowed with care. Rep hands often look thicker and lose that open feel. Date fonts run wide, and the window may sit a hair off center. Caseback engraving is another tell. Deep, crisp stamps on the real thing; pale laser etching on many fakes.

Movement is the whole game

Most 3817 replicas run an Asian 7750 or 7753, or a hand-wound chronograph with a different subdial layout. The 7750 family gives you a working chronograph with a 30 or 60 minute counter, but the flyback is not real. The pusher may reset the running chrono, which feels like a flyback, yet the mechanism is not the same and repeated use is hard on it. The 24-hour subdial is the other weak point. On some reps it is slaved to the hour hand and moves once an hour with a visible jump. On others it is a printed disc that never moves at all. Ask for video of the subdial running before you commit.

A quick checklist before you pay

• Ask the seller to demonstrate the 24-hour track advancing, not just sitting there.
• Request a video of the chronograph running, then a reset while it runs.
• Count the fluting segments and look at depth consistency around the band.
• Check crystal clarity from an angle. Heavy blue haze means cheap coating.
• Confirm the crown screws down and the pushers feel firm, not mushy.
• Weigh it. A 42mm steel chrono on leather should feel solid.
• Look at the caseback under a light. Sharp engraving, or shallow scratches?

Price tiers, in plain terms

Entry builds under $200 get the shape right and not much else. The movement is crude, the crystal is hazy, and the bracelet or strap feels thin. Mid tier, around $250 to $450, is where the good value sits: better case finishing, proper AR coating, a serviced 7750, and a dial that holds up in daylight. Above $500 you are paying for finishing details and a movement that has been regulated. That is a lot for a replica, and returns are thin.

Living with one

Do not swim in it, whatever the listing says. Gaskets on replicas are a coin flip, and a flooded chronograph is scrap. Have it serviced if the chronograph starts running slow or the reset hand drifts off zero. Keep it off a winder, and store it somewhere dry. None of that is different from owning any mechanical chronograph.

Buy the look if the look is what you want. Buy the watch if you want the flyback, the finishing, and the service history behind it. Those are two different purchases, and the 3817 makes that distinction easy to see once you have both in hand.