Official Helmet of the British Navy, the Three Light, Six Bolt Siebe Gorman Diving Helmet was introduced in 1915 and stayed in production through World War II. This example, circa 1930, has the “dove-tail” finish to the manufactured top of the lid. Numbered 13660 on the interior. Maker’s Plate reads “Siebe Gorman & Co. Ltd, Submarine Engineers, London, Patent. With a custom built wood display stand, and an original key for the bolts, which is quite rare.