Short answer
131.20 is the material block in an Omega reference number. On the modern Constellation line it means a two-tone watch: a stainless steel case and bracelet with gold on the bezel, crown, and center links. A watch sold as a "Constellation 131.20" is fake when the full nine-digit reference does not exist in Omega's catalog, when the case and bracelet are all steel, when the "gold" is plating, or when the engraving, dial, and movement do not match the reference they claim to be.
What the 131.20 block means
- 131 is the Constellation family code for the current generation.
- 20 is the material slot: two-tone steel and gold case and bracelet. The same slot read as 10 means all steel.
- The remaining groups cover case size, movement, dial, and variant, so a full reference reads like 131.20.XX.XX.XX.XXX.
- There is no single "131.20 watch". Two watches sharing that block can differ in size, dial color, and stone setting.
How to check one, step by step
- Read the full reference engraving on the caseback and write down every digit.
- Compare all nine digits against Omega's published reference list for the Constellation line.
- Measure the case across the bezel with calipers and match the result to the size block in the reference.
- Weigh the watch on a kitchen scale. A two-tone 39 mm Constellation on a full bracelet sits near 130 to 150 g; a plated copy with a hollow bracelet comes in 20 to 40 g lighter.
- Study the gold sections. Genuine two-tone Constellations carry solid 18k gold on the bezel, crown, and center links, so the color runs through the metal. Plating shows a grey or brown edge at wear points.
- Check the dial print under a loupe. The Constellation star and the word CONSTELLATION sit in sharp relief, and the date window edges are cut clean.
- Turn the watch over and inspect the movement. A genuine Master Chronometer calibre shows factory engraving, a free-sprung balance, and a serial that matches the paperwork.
- Inspect the bracelet and clasp. Genuine end links meet the case with no gap, the clasp closes with a solid click, and the link pins are screws rather than friction pins.
- Match the serial at the 7 o'clock lug and on the movement to the number on the warranty card and pictogram card.
Red flags
- Reference digits that break Omega's numbering pattern, such as a 20 material block on an all-steel case.
- Dial print that is fuzzy, a star that is off-center, or a date window with rough edges.
- Gold that scratches to a silver or grey tone under the bezel and clasp.
- A caseback with shallow laser etching instead of deep stamping or engraving.
- A rotor with printed text instead of engraved text, or a movement number that does not match the card.
- Box, cards, and serial that arrive from three different sources.
Where replica 131.20 watches sit
Replica Constellation 131.20 watches range from cheap plated copies to high-grade builds with cloned movements. The tell is not the logo. It is the metal and the calibre. A replica can copy the dial layout and the caseback print, but it cannot copy solid 18k gold or a METAS-certified movement at replica prices. If you buy a replica, judge it on case finishing, bracelet fit, and whether the plating survives daily wear, and treat any claim of "1:1 gold" as plating until a jeweler tests it.
Bottom line
Treat 131.20 as a material code, not a model name. Verify the full reference, the metal, the weight, and the movement serial before you accept any Constellation as genuine, and expect replica versions to fall short on the gold and the calibre rather than on the dial print.