https://www.analogshift.com/products/seiko-6309

In 1976, Seiko released the Reference 6309. Like its predecessor, the 6105, the 6309 has a cushion case. However, the shape of the case was something entirely new for Seiko—a 42mm symmetrical design reminiscent of a turtle’s shell.

This shape has become nothing short of iconic for Seiko, and is still present in their collection today.

The dial on the Seiko 6309 is a masterpiece and, like several other dive watches, is a lesson in simplicity. The famous “sword” hour indices at the poles are complimented by circular lume dots throughout. Other improvements included a day function and a thicker bi-directional bezel with two rings of knurling versus one, making it easier to operate with gloves.