Mention high-end Seiko, and relative newcomers to Seiko will assert Grand Seiko as the be-all end-all, but experienced Seiko hands will always mention King Seiko – like full-serviced 1969 King Seiko 5626-7040 "Esso" Chronometer automatic dress watch here, one of the first King Seiko automatics made – in the same breath.  Seiko phased out the King Seiko in the 1970’s, virtually ensuring the Grand Seiko would remain more famous
 
Why do we call this King Seiko, "Esso"?
 
Esso is a trading name for ExxonMobil – yes, that ExxonMobil.  Originally, the Esso name was primarily used by its predecessor Standard Oil of New Jersey after the breakup of the original Standard Oil company in 1911.  The company adopted the name "Esso" from the phonetic pronunciation of Standard Oil's initials.
 
Standard Oil of New Jersey started marketing its products under the Esso brand in 1926. In 1972, the name Esso was largely replaced in the U.S. by the Exxon brand after the Standard Oil of New Jersey bought Humble Oil, while the Esso name remained widely used elsewhere.  However, in most of the world the Esso brand and the Mobil brand are the primary brand names of ExxonMobil, while the Exxon brand is used only in the United States alongside Mobil.
 
In Japan specifically, Exxon has had a long history beginning operations in 1893 selling kerosene and lubricants.  Esso was established as Esso Standard Sekiyu K.K. in 1962, following the dissolution of the Standard Vacuum Oil Company and subsequently became Esso Sekiyu K.K. in 1982.  
 
After the Exxon and Mobil merger in 1999, the Japanese subsidiaries were reorganized as ExxonMobil Y.K. in 2002, which spun off its downstream business to EMG Marketing G.K. on 2012, and acquired as a subsidiary by TonenGeneral Sekiyu K.K. in the same year.  In 2016, JX Holdings and the TonenGeneral Group merged into JXTG Holdings (now Eneos Holdings), leading to the dissolution and absorption of EMG Marketing into a subsidiary of the new company, JXTG Nippon Oil & Energy (now Eneos), in 2017.  
 
In 2019, ExxonMobil began to phase out the Esso and Mobil brands in Japan, replacing it with JX's Eneos EneJet banner with approximately 2,800 Esso/Mobil service stations rebranded.  While the retail fuel brand is gone, ExxonMobil continues to operate in Japan, particularly focusing on high-margin businesses like chemicals, LNG marketing, and marketing Mobil lubricants rather than large-scale refining.
 
Grand Seiko, King Seiko, and the "Grammar of Design" 
 
In 1959, Seiko split up their Suwa subsidiary into two separate entities—Suwa Seikosha and Daini Seikosha—to promote competition and product development within the company, with both operating separately under the idea this would incite competition and each would try to one-up each other and produce better products.
 
Well…it worked.
 
That same year, Daini Seikosha hired a young designer by the name of Taro Tanaka, the man who would in the early 1960's create a set of design principles that he called “The Grammar of Design.”   In 1962, Tanaka noticed Swiss watches "sparkled brilliantly" and realized the design of high-end Seiko watches could be radically improved through the implementation of "flat and conical surfaces perfectly smooth and free of distortion."  
 
This "Grammar of Design" was implemented in Grand Seiko and King Seiko lines from 1967 and made these lines instantly recognizable as status symbols in the hierarchical Japanese business world of the 1960s and 1970s.
 
Tanaka’s rules would go on to fundamentally change Seiko’s design language.  All surfaces and angles of the case, dial, indices and hands had to be flat and geometrically perfect to best reflect light.  Following this aesthetic, the bezels were to be simple two-dimensional faceted curves.  And third, no visual distortion from any angle was allowed, and all cases and dials had to be mirror-finished.  In “A Journey in Time: The Remarkable Story of Seiko,” Tanaka’s approach to the new style is described as follows:

“He started by creating cases and dials that had a perfectly flat surface, with two-dimensional curves on the bezel as a secondary feature.  Three-dimensional curves were not used, as a general rule.  He also decided that all distortion should be eliminated from the dial, too, so that it could be finished with a mirror surface.  This formed the basis for the new Seiko style.”
 
The 5626’s contoured case is reminiscent of the legendary 1967 Grand Seiko “44GS” but perhaps even more attractively rendered.  Like the 44GS, the 56KS features a beveled ring around the crystal that sits above the sharply-sculpted and polished body and lugs.  That said, the 56KS differed with its long, elegant lugs that draw attention to the dial rather than the case. 
 
The long, thin applied chiseled hour markers haven’t been duplicated by Seiko or Grand Seiko since the 5626, standing out in contrast to the fatter markers found on the 44GS and modern watches.  Even more unique, the King Seiko eschews the trademark sword hands of the Grand Seiko line for impossibly-thin pencil hands that complement these fine markers.
 
Now, on to the chronometer part.  For a brief period in the late 1960’s and early 1970’s, some Seiko’s were certified chronometers by Bureaux officiels de contrôle de la marche des montres (precursor to COSC).  Seiko's history with the Swiss official chronometer rating is an interesting one, from Seiko's first participation in 1963 to its entry of the 45 calibre here in 1968.
 
Seiko nearly always placed rather well vis-a-vis its Swiss competition - almost certainly to the embarassment of the Swiss.  When the successor to the chronometer contest was established, the COSC standard, the Swiss made the odd decision that "all parts used to build the movement must be made within Europe”...jealous much? 
 
Seiko would eventually eschew the Swiss-based chronometer certification in favor of their own, more stringent, standard, thusly beating the Swiss at their own game.  And Seiko would go on to be the first to invent the automatic movement...but that's another story.  King Seiko production lasted loosely from 1968-1974, with the Seiko introduction of its Astron, the first quartz watch, in 1969 ultimately spelling the death kneel of the KS high frequency movement.
 
This King Seiko comes on a vintage stainless-steel bracelet, and with nylon strap, rugged travel case, and springbar tool.