Cal. 4S12 is an automatic movement with date and 24 hour hand developed by Seiko in the early 1990s. It was based on the Seiko 52 stream of movements, which had been retired in 1976, and was responsible for Seiko’s resurgence as a maker of high-end watches. Cal. 4S12 is very similar to Cal. 4S15, which lacks the 24 hour hand.
The 24 hour hand is located centrally on the dial, coaxial with the ordinary timekeeping hands. It is not adjustable separately from the 12 hour hand. This leads some to consider it not to be a “true GMT”, though this is the same function as the famous Rolex GMT-Master.
Cal. 4S12 was used in the SUS GMT military watch, which included an internal rotating bezel to enable the user to “set” the GMT hand for a second time zone.
- Seiko SUS SCFF005
- Seiko SUS SCFF007
- Seiko SUS SCFF009
Early Seiko 4S Movements
From the 1992 introduction of the Seiko 4S family through the early 2000s, Seiko used the 4S line as a high-end movement for diverse watches, including the SUS military, Alpinist, and Titanium diver. Most of these models were retired shortly after introduction, and the series of simple 4S movements was phased out by 2003.
| Production | Watch | Winding | Jewels | Notes | |
|---|---|---|---|---|---|
| 4S12 | 1997-1998 | SCFF | Automatic | 25 | Date Fixed 24 hour hand |
| 4S15 | 1995-2001 2006 | LHAM SCFF SCVF SCVN | Automatic | 25 | Date |
| 4S24 | 1990s | LJAL SCVG | Manual | 21 | |
| 4S25 | 1993-1998 | SCVH SCVJ | Automatic | 25 | Date |
| 4S28 | 1994-2000 | LJAK SCVG SCVM | Manual | 24 | Small seconds at 6:00 |
| 4S35 | 1992-2000 | SCVK | Automatic | 25 | Date |
| 4S71 | 1995-2003 | GCBB | Automatic | 25 |