Late 1950’s Gruen Precision Diver aka Ocean Chief - cal. 560 RSS. These share many traits with the Breitling SuperOcean, including a beautiful concave bezel.
I love this simple 1960 UG... Its the only one of my watches that just looks good on any strap and at any time...even on this fake rolex bracelet that I found in a box of random junk and only put on it for a laugh!