The short answer

The Cartier Tank Must that Cartier revived in 2021 is one of the most copied new watches on the market. A steel case, a quartz movement in the two smaller sizes, and a shape that has barely changed since 1917 make it cheap to clone and hard to police. A fake can look convincing in a listing photo. In hand, most of them fail on four things: case thickness, dial printing, the crown cabochon, and the caseback engraving.

What the real 2021 Tank Must actually is

Cartier relaunched the Must name in 2021 as an entry point to the Tank line. The family runs across three sizes. The Small measures about 29.5 by 22 mm, the Large about 33.7 by 25.5 mm, and both use a quartz caliber (Cartier 157) with no seconds hand. The XL is roughly 41 by 31 mm, automatic, thicker, and carries a date at 3 o'clock. The quartz cases sit around 6.6 mm thick, which is thinner than most people expect.

Details worth memorizing: silvered opaline dial, Roman numerals, blued steel sword hands, a secret Cartier signature worked into the VII numeral, and a blue synthetic spinel cabochon set in the crown. The SolarBeat version swaps the battery for a photovoltaic movement that Cartier rates for roughly 16 years, and it ships on a strap made partly from apple waste rather than leather.

Why this reference gets copied so hard

Quartz movements are the reason. There is no decorated automatic caliber to fake, no exhibition caseback to fool anyone with, and no seconds hand sweeping in a way that reveals a cheap movement. A replica maker only has to get a rectangular steel case, a printed dial, and a two-hand quartz module right. That is a much lower bar than a Santos or a Ballon Bleu with an in-house automatic inside.

Where fakes give themselves away

• Thickness. Genuine quartz Tank Must cases are thin. Most replicas run half a millimeter to a full millimeter thicker because the case is built around a generic movement and a spacer. Put a caliper on it, or compare the side profile to a photo of the real thing.
• Dial printing. Cartier's Roman numerals are drawn with fine, even strokes and specific serifs. Replicas tend to print heavier numerals with clogged counters, a flat or muddy opaline finish, and hands that stop short of the minute track. Look at the VII under a loupe: the secret signature should be there and should be crisp.
• The cabochon. This is the fastest tell. The genuine stone is domed, smoothly polished, and a deep blue. Fakes use plastic or glass that reads flat, cloudy, or purple under light, and the setting often sits proud of the crown instead of flush.
• Caseback and hardware. Engraving on a real Tank Must is sharp and shallow, with correct reference and serial formatting. Fakes use laser etching that looks sandy up close, or stamped text with soft edges. The buckle and strap hardware matter too. Genuine pin buckles are properly finished and stamped.

The SolarBeat curveball

The SolarBeat Tank Must hides its photovoltaic cells behind a translucent dial, so the dial has a slightly different depth and light behavior than the standard opaline version. Replicas of SolarBeat pieces are almost always just standard quartz watches with a SolarBeat dial and strap thrown on. There is no way to tell from the outside without opening the case, which is one more reason to buy from someone who will let a watchmaker look.

What replicas do get right

Case shape, most of the dial layout, and overall presence. At arm's length on a wrist, a decent replica reads as a Tank. That is exactly why the resale market has a problem: these watches get flipped as genuine to buyers who never handle the real thing. In the US, selling a counterfeit as authentic is fraud, and trafficking in counterfeit goods carries real penalties. Replica sellers who are upfront about what they are selling occupy a different lane than someone passing a fake off as the real thing.

If you are buying secondhand

Ask for a caliper measurement of case thickness, a macro shot of the crown, and a clear photo of the caseback engraving. Compare the numerals against Cartier's own product images. Price is the biggest filter of all. A 2021 Tank Must Large in steel trades in the low thousands, and anything far under that is either damaged, stolen, or fake. When in doubt, pay a watchmaker for ten minutes of bench time. It is cheaper than a mistake.