Short answer
Cartier does not make a 31mm Tank Must. The catalog runs Small at 29.5 by 22mm, Large at 33.7 by 25.5mm, and XL at 41 by 31mm. When a listing says "Tank Must 31mm," it is seller shorthand, a rounded measurement, or a case that has been fudged. The only place a 31 shows up is the XL width, and the XL is a big watch. If the photos next to that spec show something small and delicate, the number is wrong.
Get the size straight first. It is the easiest claim to check and the easiest one for a seller to blur.
What the real Tank Must is
Cartier relaunched the Tank Must in 2021 as the entry point to the Tank line, replacing the Tank Solo. It is steel, it is slim at about 6.6mm thick, and it runs a quartz movement rather than a mechanical one. That last detail matters more than anything else when you are checking a watch against a photo.
- Case: steel, rectangular, polished bezel with brushed sides on the brancards
- Crystal: sapphire
- Dial: silvered or lacquered, Roman numerals, chemin de fer minute track, blued steel sword hands
- Crown: faceted synthetic spinel cabochon, set flush to the case
- Movement: Cartier quartz caliber 157, with a SolarBeat variant on some references
- Strap: leather or steel bracelet, signed hardware
Why the sizing claim matters
A Tank case is a rectangle, so there are two numbers to talk about: the width across the case and the length from lug to lug. Sellers mix them, and the word "31mm" gets used for cases that are not that size in either direction. Ask for a caliper photo of the actual watch with the caseback visible, and compare it against Cartier's published dimensions. A seller who will not take that photo is telling you something.
Tells that separate a genuine Tank Must from a fake
- Seconds hand. The genuine watch is quartz, so the seconds hand steps once per second. A smooth sweep means an automatic movement is inside, and Cartier does not put one in the Tank Must. This is the single most reliable check in a video or live photo.
- Crown cabochon. Real ones are faceted synthetic spinel, seated flush. Fakes use glass or plastic, often too domed, too dark, or set crooked.
- Dial printing. Roman numerals should have even stroke weight and clean serifs. Look at the spacing in "CARTIER" and at the VII and XII. Fuzzy or heavy print is a giveaway.
- Caseback engraving. Genuine engraving has depth and crisp edges. Laser etching looks shallow and slightly gray under raking light.
- Finishing. The polished brancard edges should be sharp against the brushed sides. Fakes round that transition off or polish the whole case.
- Strap and buckle. Signing, font, and stitch spacing should match the case era. Mismatches are common.
Sapphire versus mineral glass is hard to confirm by hand, so lean on the movement and the cabochon instead of a water drop test.
The legal side in the US
Selling counterfeits is a federal crime under 18 U.S.C. 2320, and US Customs seizes counterfeit watches at the border. A fake also means no Cartier service, no warranty, and a watch you cannot resell honestly. That is worth knowing before a $200 purchase turns into a $2,000 problem.
If you are comparing before you buy
- Pull the official dimensions from Cartier and write them down.
- Ask for a caliper photo and a caseback photo, not stock images.
- Ask what movement is inside. The answer should be quartz.
- Request a close crop of the crown cabochon and the dial print.
- Buy only from a source with a clear return window.
Do those five things and most of the bad listings drop out on their own. The 31mm label is usually the first thing that does not add up, and it rarely turns out to be the last.