The Cal. 51 series is a short-lived family of automatic movements produced by Daini Seikosha. It was used in the Seiko Five, Seikomatic P, and Presmatic watch lines from 1967 until 1971. It was phased out in favor of the more advanced Daini 52 stream starting in 1970.
The first automatic movement designed by Daini Seikosha, the 51 stream has many unusual features. Most notable was the quickset date, which was operated by a push button in the crown. This gave the “Presmatic” its name and was common to all movements in the series. The design was also generous with jewels, ranging from 23 to 33.
It lacked some signature Seiko features as well. The day wheel was mono-lingual, unlike the follow-on 52 stream. And it used a conventional reverser for the automatic winding rotor rather than Seiko’s Magic Lever system.