were to designate between those put into a regular case (open thru back) and the "1-piece" case (open thru front). On the "1-piece" case there was an adjuster screw placed on the exterior of the case [between the lugs] that allowed the watchmaker to adjust the movement without having to crack open the case [thru the front!]. I am not aware of a "C" variant of 5626. In other lines the letter variants were also used to indicate design changes from one "generation" of the movement to the next. For example, the 5722A Grand Seiko had a "tadpole" type fine regulator, while the 5722B had the "rack & pinion" style (as well as a higher beat-rate).
Also keep in mind that the 4th digit of the calibre # was used (mainly by Suwa division, but to a large degree by Daini as well) to designate a combination of the type of display and winding type. Example: a "6" means an automatic-wind with day & date, while "5" means automatic-wind with date only, and "1" means automatic-wind with no calendar. This explains the difference between [for example] a 5621 King Seiko, a 5625 KS, and a 5626 KS.
My research shows [in large part from conversations with Tokunaga-san's son, Tachy] that the 3rd digit of the calibre number was [during the time period in question] used as the indicator of "quality" or "accuracy" level of the watch movement - in certain limited situations. In the mechanical Grand Seiko movement family it was a "formal" hierarchy where the higher the 3rd digit, the higher the accuracy grade of the movement [from "AA" up to "AAAA"]. In the 52 & 56 King Seiko [and it's variants & related models] the usage was not so formal, but there was some informal use of that digit. But, the "Special" designator on the dial was used [according to some] to indicate that the movement was adjusted to 1 accuracy grade higher than in the "normal" models using the same movement line. So, anecdotal information says that while a 5246 King Seiko was an "A" grade watch, the 5246 or 5256 KS [and KS VANACs] with "Special" label on dial were actually "AA" grade, same as an entry-level GS.
In addition, the 5626 was used in the King Seiko, KS VANAC, and "Officially Certified Chronometer [with or without KS designation] models, but the same [design/architecture] movement, but called 5646 - and better adjusted - was used in the 56 GS line.
If you do have any source info showing the letter suffixes being used as some kind of a quality indicator, I'd love to see it! That would be a new piece of info (to me) on Seiko's calibre # "code" that I'd not been able to find before.
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