https://www.analogshift.com/products/omega-seamaster-300-big-triangle

Omega adapted the Seamaster of the 1940s, adapting a technology from submarines—rubber O-rings—to seal the back of the case.

Sure that this was a winning design, Omega submitted this new Seamaster to the Laboratory for Water Resistance in Geneva, where it was subjected to the same conditions it would see during a dive: punishing pressure, the same as at 60 meters, and rapid changes of temperature. It passed, and the Seamaster was rechristened the Seamaster 300 and launched in 1957. As Omega’s answer to its longtime rival Rolex’s Submariner, the Seamaster found its depth.

But it did not stagnate—as underwater breathing technology continued to improve, Omega tweaked the design of the Seamaster throughout the remainder of the 1950s and on to the 1960s, and hit on a legend.

This watch, the Reference 166.024, is like its cousin, the Reference 165.024, but with an additional date window. Like the Speedmaster Reference 145.012, the case of the Seamaster 166.024 was made by Huguenin Frères, and is notable for its distinctive asymmetrical shape. The dial is also of note, due to its unusual configuration: a large triangle at 12 o’clock which has given rise to the nickname “Big Triangle.”