The 4S15 SUS military watch is a fan favorite in the Seiko watch community.
This caliber movement is a refinement of the chronometer grade 5246 movement introduced in the early 1970’s (King Seiko Vanac). The movements are mechanically very similar and seem identical.
The 4S12 as a high-end movement was adjusted to +15/-10 seconds per day from the factory. One can compare it to the top grade ETA 2892 movements.
As in the 4S15 it features a GMT hand slaved to the hour, a thermo-compensating balance and hairspring, and hacking and handwinding of course.
Cal. 4S15 was used in Seiko SCVK001, SCVK002, Brightz (SAGN), Laurel (LHAM), Red Alpinist (SCVF), Titanium Divers, SUS Military and King Seiko LE (SCVN001) from 1992-1997 and in 2000.
Caliber 4S12 was mainly offered in another SUS a very attractive "civilian edition" with the obiqutous non-independent GMT. Out of necessity the GMT models differ in the design and utilization of the chapter ring. The military chapter ring marks out the minute and seconds while the GMT it is a rotating 24 hour ring operated by the 2nd crown at the 4 o’clock position.
There are just two SUS mechanical models powered by the 4S15 and the 4S12 movements respectively.
The quartz edition however with all possible complications down to leap years was aimed at the student - here the Simple and Strong of SUS actually could best be interpreted to mean a Steel and Sturdy case that would just not sit modestly on the wrist. The SUS never was a wall flower watch.
The 4S12 civilian models (JPY 38,000) and the student's SUS (JPY 38.000) were more expensive than the military 4S15 models (JPY 30,000) and more flashy for sure, they came in three color options, the aquamarine metallic blue SCFF009, the black SCFF005 and the red metallic sunburst SCFF007. All have Seiko's hardlex fitted (on the case back also) and all are lumed with Lumibrite but interestingly the application of it can be uneven, one may surmise that even in the nineties not everything in the Seiko factory had been automated.