Short answer
220.12.41.21 is an Omega reference prefix. It covers the Seamaster Aqua Terra 150M Master Chronometer with a 41 mm stainless steel case and calibre 8900. A watch sold as a fake Aqua Terra 220.12.41.21 is a counterfeit of that reference. Omega does not make it. The final digit groups of a full reference, such as 220.12.41.21.03.001, set the dial color and the bracelet or strap choice.
Reading the reference
- 220: Seamaster Aqua Terra 150M product family.
- 12: stainless steel case.
- 41: case diameter in millimeters.
- 21: movement group, here calibre 8900.
- Last groups: dial variant and strap or bracelet.
Omega publishes each full reference in its catalog. The prefix alone does not name one single watch.
Genuine spec
- Case: stainless steel, 41 mm wide, about 13.2 mm thick.
- Lug width: 20 mm.
- Water resistance: 150 m.
- Crystal: sapphire front, sapphire display back.
- Dial: horizontal teak lines, date window at 6 o'clock.
- Markers and hands: applied metal frames with Super-LumiNova fill.
- Bracelet: steel with a butterfly clasp. Rubber strap versions exist.
Calibre 8900
- Automatic, twin barrels in series, 60 hour power reserve.
- 25,200 vibrations per hour, or 3.5 Hz.
- 39 jewels. Co-Axial escapement.
- Silicon balance spring. Free-sprung balance with no regulator index.
- Magnetic resistance to 15,000 gauss.
- METAS Master Chronometer certification across eight tests.
Where replicas differ
- Beat rate. Most replicas use a 28,800 vph base, such as an ETA 2824 clone, a Miyota 9015, or a Seagull ST21. A timegrapher that reads 28,800 under a rotor stamped 8900 is a clear tell.
- Power reserve. Clones stop near 38 to 45 hours. The genuine movement runs 60 hours.
- Balance. Genuine parts use a silicon hairspring and no regulator arm. Clones use a metal hairspring and a regulator.
- Rotor and bridges. Replica decoration is stamped or laser etched. Genuine arabesque Geneva stripes are cut, with blackened screws.
- Date wheel. Font weight and centering differ from the genuine part.
- Case finish. Genuine lug bevels are polished and sharp. Replicas show soft edges and wider gaps where the end links meet the lugs.
- Crystal. Replica anti-reflective coating is thinner. Reflections look different at an angle.
- Serial numbers. Genuine case and movement numbers match. Replica numbers repeat across units.
Checks before payment
- Request a timegrapher photo showing rate, amplitude, and beat rate.
- Request a clear photo of the balance wheel and the hairspring.
- Compare caseback engraving depth against a known genuine example under a loupe.
- Confirm the reference number on the caseback matches the papers.
- Ask for service history. Omega does not publish a public serial database, so documents and inspection carry the weight.
Omega does not list a weight for this reference. Compare against a known genuine watch on a scale instead of a number from a listing.
Legal note
US law treats trafficking in counterfeit goods as a federal crime under 18 U.S.C. 2320. US Customs and Border Protection seizes counterfeit watch shipments at the border. A counterfeit carries no Omega warranty and no service support at an Omega service center. Resale value is near zero.