Seiko’s first range of truly submersible sports watches came in the form of the Seikomatic 50m SilverWave in 1961, which marked a number of firsts for Seiko – it represented the Japanese company’s first dive watch; their first watch with an inner rotating countdown bezel; their first use of the tsunami wave symbol; and it was their first automatic watch with a screw-down case back.
 
Fast forward to the ever more refined 1977 Seiko 6306-8000 Silverwave "Mitsui Toatsu Chemicals" automatic diver here on its original bracelet, which has lost some of the early diver’s rough edges yet retained all (and then some) of the charm.
 
But why do we call this a "Mitsui Chemicals" diver?
 
Engraved on the watch's caseback in Kanji Japanese is the phrase, "30 Years of Continuous Service, Mitsui Chemicals," marking this as a commemorative award to a loyal employee of three decades at Mitsui.
 
Mitsui Chemicals (三井化学株式会社, Mitsui-Kagaku Kabushiki-gaisha) is a Japanese chemicals company listed on the Nikkei Exchange with business interests in Japan, Europe, China, Southeast Asia and the United States with nearly 14,000 employees and over $12 billion USD in revenue.
 
Founded in 1968 as Mitsui Toatsu Chemicals after a merger of Toyo Koatsu (1933) and Mitsui Chemical Industry (1941), the company is one of the leading chemical companies in Japan and part of the Mitsui Group conglomerate.  The company mainly deals in performance materials, petrochemicals and basic chemicals and functional polymeric materials - to include, interestingly enough, dye used in the manufacture of jeans during and after the Japanese craze for anything denim (see W. David Marx's excellent, "Ametora: How Japan Saved American Style" for more on this fascinating era in Japanese and American history).
 
Mitsui Chemicals is part of the much larger Mitsui Group (MG, 三井グループ, Mitsui Gurūpu), a Japanese corporate group and keiretsu that traces its roots to the zaibatsu groups dissolved after World War II.  Unlike the zaibatsu of the pre-war period, there is no controlling company with regulatory power – instead, the companies in the group hold shares in each other, but are limited to exchanging information and coordinating plans through regular meetings.
 
Founded in 1876, following WWII the Supreme Commander Allied Powers pressed the Japanese government to dismantle the ten largest zaibatsu conglomerates, including MG, which was broken into many separate companies, reorganized itself as a horizontal coalition of independent companies in the 1950s, once the occupation of Japan had ended and some of the smaller companies were allowed to re-coalesce. 
 
MG has moved from strength to strength since, and now counts over 500 unique companies involved in banking and insurance, steel and chemical production, engineering and shipbuilding, oil and shipping, among a slew of other industries.  It has a presence in 62 countries (126 offices and nearly 47,000 employees) with nearly $115 billion USD in assets and $107 billion in revenue in 2023.
 
As for the Seiko 6306 diver itself?  Between 1965 and 1975, Seiko brilliantly conceived, designed, and produced three generations of classic ISO-certified 150 m divers’ watches, in chronological order - the 62MAS, 6105-8000/9, and the 6105-8010/9.  Then 1976 arrived...
 
That year, Seiko debuted its replacement for its 6105, conceived in two different specifications - one of which was the 6306 for the domestic Japanese Domestic Market and the 6309 for the international market; the JDM variant - the Calibre 6306 - was fitted with a more advanced 21 jewel 6306A movement, with hacking and an English/Kanji Japanese day wheel.
 
The 6306 lived a rather short life in between the 6105 and down-jeweled and less complex 6309, and was produced from 1976 to 1981, when Seiko would favor the 6309 until it too was replaced with the 7002 diver in 1988. The 6306 calibre was a successful effort by Seiko to improve the 6105 movement, which allowed for greater lifespan, robustness, and ease of maintenance.
 
This Seiko 6306 automatic diver comes on its original Seiko-signed bracelet, and with nylon strap, rugged travel case, and springbar tool.