Within the world of vintage Seiko, Grand Seiko – distinguished by the “GS” on dial/crown and gold medallion on its caseback, as seen on this crisp dark blue-dialed 1971 Grand Seiko 5645-7010 automatic here – inhabits the top of the food chain, simple as that. Grand Seiko demonstrates well what's so incredible about Japanese craftsmanship, namely, supreme attention to detail and refinement.
 
Grand Seiko – with 51 different models – were simply the most painstakingly designed and exquisite watches made by Seiko, produced by both the Dani and Suwa factories, including the 56GS line here.
 
The 56GS series, often referred to as “Ultra Thin,” included 18 variations - and transitioned from manual wind to automatic beginning in the 1970s.  That said, precision level suffered as a result, but reliability level increased with its balance of maintenance, reliability, and precision.
 
The Grand Seiko 56GS series is the final Grand Seiko model Seiko made before production ceased in 1976, a self-inflicted fatality of the Quartz Crisis (which was brought on by the Seiko invention of the quartz watch in 1969).  Produced from mid/late 1970 through circa early 1976, based upon the serial number, this is an earlier model produced in mid-1972.  The GS 5645 is regarded as one of the best combinations of reliability, precision, and maintenance within the original GS lineup.
 
Of note, its relatively uncomplicated to get parts for the 56GS, despite over half a century of time passing since its production - a slightly less fine-tuned calibre 56 movement was also used in King Seiko and Lordmatics. 
 
Make no mistake, Grand Seiko marked - and still continues to mark - the very best Seiko has to offer, easily on par with the vast majority of what Switzerland has to offer by way of competition.
 
Speaking of Grand Seiko design, in the early 1960s, Seiko’s Tanaka created a set of design principles 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."
 
Tanaka’s Grammar of Design made the Grand and King Seiko 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, 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.”
 
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 in 1968.
 
Seiko nearly always placed rather well vis-a-vis its Swiss competition - almost certainly to the embarrassment 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 chronograph and then the world's first quartz movements (beating the Swiss, again, both times)...but those are stories for another time.
 
This Grand Seiko comes on a stainless-steel mesh bracelet, and with nylon strap, rugged travel case, and springbar tool.