Introduced in 1969, alongside notable similar movements from Zenith and Chronomatic and before the Lemania 5100 and Valjoux 7750, the Seiko was the first chronograph to feature a vertical clutch mechanism, an advance now found on many high-end chronographs. It was not a luxury movement, however, and was used in many utilitarian and tool watches, including a “speed timer” and diver.
The 6138/6139 is related to the Seiko 6100 calibre family, sharing some parts.
The 6138/6139 lack a running seconds hand. The central sweep seconds hand is only used for the chronograph function and there is no small seconds subdial. The day and date quickset mechanism is unusual for Seiko in that it is a crown pusher type The date is set with a light press of the crown, while the day is set with a deeper press.
In 1973, astronaut and former Thunderbird pilot Col. William Pogue, carried his personal Seiko 6139 into space on the Skylab 4 mission. This yellow-dialed watch accompanied an official-issue Omega Speedmaster, but he preferred the “speed timer” feature of the Seiko to time engine burns.
This became the first automatic chronograph in space, and the watch sold at auction in 2008 for $6,000.
There were two versions of the 6138/6139 chronograph produced. The later “B” movements feature a modified switching mechanism.
All feature quick-set day and date.