Rolex made for Officine Panerai. An extremely rare and large stainless steel cushion-shaped diver's wristwatch with early unsigned dial

Officine Panerai
Officine Panerai was founded in 1860 by Giovanni Panerai, and became known for specializing in the production of highly precise optical and mechanical instruments, including compasses and depth gauges. They eventually became the official supplier to the Royal Italian Navy.
Rolex made for Officine Panerai Reference 3646
In the 1930's, the Royal Italian Navy approached Panerai with the request of designing a watch resistant to extreme conditions while at the same time keeping exact time. The prototype that Panerai submitted was called "Radiomir" and successfully passed all tests. From 1938 until the 1950's, a small series of reference 3646 watches were produced, mainly for the Italian Navy. The models were fitted with hand-wound movements and waterproof cases supplied by Rolex. The dials consisted of two discs, the lower one coated with "Radiomir" luminous substance visible through the cut out numerals in the black upper disc.
The present watch is from the so-called "no-name" series with blank dial, which were genrally reserved for the German Navy's Special Forces. These particular watches often have an unusual type of lattice engraving on the back, some of which bore the word "Kampfschwimmer", or "combat swimmer", referring to Germany's commando frogmen, the only special-purpose force of the German Navy. The present example is engraved "Kampflehrer", meaning "force teacher", perhaps belonging to a higher-ranking official in charge of the unit.