That string is not a Cartier reference. Cartier identifies the Tank by family and model: Tank Must, Tank Louis Cartier, Tank Francaise, Tank Americaine. Official references start with a W or sit in an older four-digit series. An eight-digit run like 8240029 usually shows up as a case serial, a dealer's listing ID, or a factory batch stamp on the caseback. Search it and you are not looking up a watch model. You are looking up one specific listing, and listings get relisted under fresh numbers all the time.
So the useful question is not what 8240029 means. It is which Tank you are being offered, and whether the details line up.
Find the size before anything else
The Tank is a small watch, and most disappointment comes from size, not finish. A Tank Must small measures roughly 29.5 by 22 mm. The large runs about 33.7 by 25.5 mm. Tank Louis Cartier sits in a similar large-case range but wears thinner and flatter. If a seller quotes you a size with no numbers, ask for a caliper photo across the case and lug to lug. A 40 mm Tank is not a Tank. It is a different watch wearing the name.
Wrist size matters here more than with a round watch. On a 6.5 inch wrist the small reads dressy and correct. On a 7.5 inch wrist it can look like a child's watch. The large is the safer pick for most men in the US.
Quartz, hand-wound, or automatic
Most Tanks sold today are quartz, which is also why replicas of them are so common. A quartz movement is cheap, thin, and easy to source, so the case and dial get the factory's attention instead.
- Tank Must and Tank Solo: quartz. No running seconds hand on the dial.
- Tank Louis Cartier: hand-wound, typically the 430 MC. Thin, no rotor.
- Some larger references: automatic, with a thicker caseback.
Here is a check that catches a lot of bad copies. The quartz Tank has no central seconds hand. Cheap replicas often add one, because the movement they use has a seconds pinion. If you see a ticking second hand on a watch being sold as a Must, walk away.
Dial and case details that decide the grade
Roman numerals on a genuine Tank are printed or applied with tight, even spacing, and the XII sits centered under the 12 position. On replicas the numerals drift. The VI crowds the date window or floats too high. Look at the minute track: it should be continuous and evenly weighted, not broken where the numerals cut in.
Older Tanks hide a micro-signature inside one of the numerals, usually the VII or the X. It should read Cartier when you zoom in with a loupe or a phone macro shot. Modern Must pieces dropped it, so its absence proves nothing. Its presence, badly done, proves plenty.
The crown should carry a blue cabochon, and the stone should sit flush. Blued steel hands look dark navy in flat light and flash blue at an angle. If they look painted, they are painted. Caseback engraving on a real Tank is crisp and shallow with clean serifs. Laser-etched replicas look sandy under magnification.
Steel, gold plate, and vermeil
Vintage Must de Cartier cases were vermeil, which is heavy gold plating over sterling silver. Modern Must is stainless steel. A replica that claims vermeil is almost always brass with flash plating. It will wear through at the lugs within a year. If the seller will not say what the base metal is, assume brass.
Strap, clasp, and how it wears
Genuine Tanks take a 16 mm to 20 mm strap depending on the case, and the spring bars sit close enough to the case that the strap bends sharply at the lugs. A thick aftermarket strap will not fold right and will push the watch off center on the wrist. Ask for a straight-on wrist shot rather than a product photo on a stand. Most references use a deployant clasp, and it should not rattle when you shake it.
What the price is telling you
Under about $60 you are buying a fashion watch with a Cartier logo on it. Between $150 and $350 is where decent quartz Tanks live: correct dimensions, sapphire crystal, clean dial printing. Above roughly $400 people are paying for automatic and hand-wound attempts, and the gap between those and a genuine watch narrows in some places and widens in others. The movement is usually where the money went, and it is also where the shortcuts hide.
A short checklist before you pay
- Case dimensions in millimeters, measured, not quoted.
- A macro photo of the dial printing and the crown.
- A movement photo with the caseback off, if the seller will take one.
- Confirmation of crystal material: sapphire or mineral.
- Strap width and whether the spring bars are quick-release.
- Return window in writing.
The part people skip
Selling a replica as a genuine Cartier is trademark counterfeiting in the United States, and US Customs can seize counterfeit goods at the border. Buying one for yourself is a different question than reselling it, and the two should not be confused. If you ever plan to move it along, describe it as a replica in the listing. Do that and you stay out of the part of this that carries real risk.
As for 8240029: treat it as a listing tag and nothing more. The watch in front of you either measures right, prints right, and moves right, or it does not.