Short answer
A Ladybird 2131 imitator is a third-party copy of a small Longines Ladybird, the ladies' line Longines introduced in the late 1950s. The "2131" part is where things get muddy. It shows up both as a vintage Longines case reference and inside the name of the later L213.1 quartz caliber. Sellers use the two interchangeably, so before you compare anything, decide which watch the listing is actually describing.
What the original watch is
Longines built the Ladybird as a cocktail watch. Cases run roughly 20mm to 24mm across and sit about 6mm to 8mm thick, which makes them feel closer to jewelry than to a wristwatch. Most examples are gold plated, gold capped, or solid 18k, with a clean dial, thin applied markers, and a slim strap or a bead-style bracelet. Movements are small, and on vintage pieces they are manual wind with a Longines signature on the bridges.
That scale is the whole point and also the hardest thing to fake well. A copy can match a photo at a glance. It seldom matches the proportions once it is on a wrist.
Case reference or caliber number
The confusion is worth naming. Vintage Longines references from the mid-century are short numbers stamped inside the caseback, and 2131 turns up in that family. Decades later, Longines used caliber designations such as L213.1 for quartz movements in its ladies' models. A listing that says "Ladybird 2131" without saying which one it means has told you almost nothing.
What I check on an imitator
- Case size and thickness. Measure it. Copies run a millimeter or two larger, because the smaller size is harder to machine.
- Lug width. Genuine Ladybird lugs are narrow, usually 10mm to 12mm. A copy with standard spring bars and a 16mm lug width is a giveaway.
- Dial printing. Look at the letterforms under magnification. Longines typography is consistent, and fakes thicken or crowd the text.
- Caseback engraving. Reference numbers, metal marks, and the Longines wording should be crisp and shallow. Laser etching looks flat and gray.
- Crown. Original crowns are small and signed. Replacements and copies sit too tall.
- Movement. A quartz copy will not carry a signed Longines quartz caliber, and a hand-wind copy will not carry a signed vintage movement.
- Plating and weight. Thin flash plating wears through at the lugs within months. Brass cases feel light and warm next to gold.
- Stones. On bezel-set examples, check whether the setting is real or glued. Glued stones drop out and leave dull adhesive behind.
The movement is the real tell
Everything on the outside can be copied. The inside is where cost shows up. Genuine L213.1 quartz watches run a small Swiss quartz caliber that is serviceable and easy to source. Vintage Ladybirds run a small manual-wind Longines movement with visible finishing. Copies tend to use generic quartz or a stripped-down hand-wind, and few of them can be serviced by a normal watchmaker without parts being made by hand. If a seller cannot say what is inside, the answer is that nothing good is.
Legal and money reality
A watch that carries the Longines name and was not made by Longines is a counterfeit. Selling one in the United States is illegal, and customs seizures happen. A homage that copies the shape without the branding sits in a different place legally, but it still is not a Ladybird.
Here is the part most buyers skip: vintage Ladybird watches trade for modest money. Small ladies' cocktail watches from big Swiss names sell well below what the gold content and the brand would suggest, because the market for them is thin. It is common to find a genuine, running example for less than the price of a well-finished copy. I would spend the search time on that instead.
Bottom line
Ask which 2131 you are being offered, a case reference or a caliber, then measure the case, read the dial under a loupe, and ask about the movement. If the answers stay vague, walk. The genuine article is small, quiet, and cheaper than most people assume.