Seiko Analogue History
In attempting to understand present day japanese analogue watches, first we must begin with the japanese manufactured movements that are currently based upon. Unless otherwise noted, all movements listed are designed and manufactured by Seiko Instruments, Inc. Note that a complete listing and description is available in the appendix of The Seiko Book by Tokuma Shoten. Since I don't have access to that volume, I have had to compile this from available sources and it may be incomplete or contain some errors on my part.
7S series calibres
These are the most prolific mechanicals produced by Seiko, and incidentally they are the only Seiko mechanical regularly imported into the US by Seiko USA. Technically the 7S calibres are very simple and efficient bi-directional winding automatic movements that have no provision for handwinding. The 21 jewel 7S26 is the most common variant, found from the US to Malaysia.
More uncommon is the 23 jewel 7S36 which is generally only seen in Japan, though ocassionally encountered in western markets. I suspect that the two extra jewels are for the auto-winding gear train. Both 7S calibres run at 21,600 vph, have day and date displays, and have a flat matte, nickel-plated finish. Unlike the rest of Seiko's mechanical movements, the 7S26 is primarily manufactured outside of Japan by a subsidiary in Singapore, though some - particularly the 7S36 variant - are manufactured in Japan.
4S series calibres
The 4S calibres make up the "bread and butter" of Seiko's mechanical movements. They are a diverse group of automatics and handwinds which are actually based on a single Seiko ebauche. It should be noted that all Seiko automatics above the 7S level can also be wound by hand.
The base movement of the 4S series is the 4S15, a 25 jewel automatic with center seconds and date calendar. It runs at 28,800 vph, has a flat brushed finish except for the rotor which is finished with colimaconnage, is nickel-plated, and has cap jewels on both the escape and fourth wheels -- a nice touch which dates back to the early Seiko "Chronometers" which often featured cap jewels all the way to the center wheel. The use of colimaconnage is also a Seiko tradition that dates back to the Grand Seiko automatic movements of the 1960s. It is probably the highest quality simple automatic movement at this price point in the world (35,000 to 45,000 Yen). Calibre 4S12 is a variant of the 4S15 with an additional 24-hour GMT hand. Calibre 4S25 is also a 4S15-based automatic.
The basic 4S handwind, calibre 4S24, is a 21 jewel center-seconds movement. Simply put, it is calibre 4S15 with the automatic winding removed. In removing the rotor and winding gears, the simple three-bridge top plate of the 4S15 is further simplified into a two-bridge 3/4 plate architecture. It has a simple flat matte and rhodium plated finish. Calibre 4S28 is a 24 jewel version of 4S24 with indirect subsidiary seconds indication. It is used primarily in Seiko's savonnette fob watches.
The 4S7x series was first introduced in 1995 for Seiko's mechanical luxury watches and is comprised mostly of automatic movements based on the 4S15. The basic movement is the 25 jewel calibre 4S71, with center seconds and no date. Calibre 4S77 adds a date subdial, retrograde day display, 24 hour GMT subdial, and three jewels. Calibre 4S79 adds a power reserve indicator, small seconds display, and four jewels. The only handwind, calibre 4S79A, is a 31 jewel certified chronometer with power reserve and small seconds display based on the 4S28. The automatic movements have a colimaconnage and gilt finish, and the handwind is finished with "Tokyo Stripes" and is also gilt.
6S series calibres
Launched in 1998, the 6S calibres are Seiko's three-register column wheel chronographs, both automatic and handwinding. Calibre 6S78 is a 34 jewel automatic chronograph with date calendar. At 7.2mm thick and 28.4mm in diameter it is a robust movement. The 35 jewel hand-wound calibre 6S74 has no date calendar but adds a power reserve display, and the 40 jewel automatic calibre 6S77 has both the power reserve display and calendar date window. The automatic movements in this series are finished with brushing on the plates and colimaconnage on the rotors, and the manual-wind is finished with Tokyo Stripes. Calibre 6S99 is the skeletonized version of calibre 6S74. All four movements are gold-plated.
68 series calibres
The 68 series is comprised entirely of thin hand-wound movements. Calibre 6810 is a 22 jewel tonneau-shaped calibre, finished with Tokyo Stripes and rhodium plating. First seen in 1993, calibre 6870 is a 21 jewel movement with only hour and minute indications. Based on the earlier 6870, ultrathin calibre 6898 is 1.98mm thick. The 6898 is finished with Tokyo Stripes, gold-plating, and a voluted ratchet wheel. Indirect small seconds display adds 5 jewels for a total of 26. Calibre 6899 is the skeletonized and engraved version of 6898 -- available gilt or rhodium plated. The 68 calibres run at a 21,600vph beat rate, and all feature cap jewels on the escape, fourth, and third wheels.
8L series calibres
The 8L calibres are very robust simple automatic movements built on the same scale as Rolex's 31xx series. The 26 jewel calibre 8L75 is 5.3mm think and 28.4mm in diameter, with center seconds and date display. It has a flat matte gilt finish, and colimaconnage on the rotor. I suspect that the 8L75 is an unadjusted and more simply finished variant of the 9S calibres. A new entry to the series is calibre 8L35 -- also featuring a date guichet and center seconds.
9S series calibres
The premium simple automatic calibre of Seiko's mechanical collection, their production is limited to 300 units per month -- a tiny number for a manufacture that makes over 30 million watches per year. The 26 jewel calibre 9S55 has center seconds and calendar date indication, and the 24 jewel calibe 9S51 is simply a 9S55 without date window. Like the 8L75 they are 28.4mm in diameter and 5.3mm in thickness. Both movements are finished with Tokyo Stripes, rhodium plating, hand-polished anglage, and are adjusted to six positions.
Additional
Seiko's analogues for the most part are not and have not been "fine" watches, but instead the "mid-tier" of practical and dependable watches based upon simple and robust movements. That they can produce fine watches has been demonstrated by a few select models - particularly their ultrathins - but the nearly 90-year history of Seiko's mechanical wristwatch production has been focused on functional and useful handwinds, automatics, and chronographs.