Seiko Caliber 6139B

Brand	Seiko
Caliber Number	6139B
Movement Type	Automatic
Jewels	17
Vibrations Per Hour	21,600 bph
Diameter	27mm
Thickness	7.1mm
Shock System	Diashock
Crown Location	3:00
Functions	Hours, minutes, central chronograph seconds, 30 minute counter, day, date
Country of Manufacture	Japan
Known Models	6139-6005 (add more in the comments below)

The crown adjusts the day/day by pushing in (lightly for day, harder for date). The crown also controls an internal rotating bezel. 30 minute subdial is at 6:00 and there is a central chronograph seconds hand.

There are different variations of this movement, including Seiko caliber 6139A, 6138A, and 6138B.

From a post on watchuseek by sempervivens:

…so Seiko cal. 6139A was produced from 1969 until 1971 and the switch from Seiko cal. 6139A to Seiko cal. 6139B took place in 1971, which lasted until 1978.

Concerning Seiko cal. 6138, everything probably happened one year later :
production of 6138A started in 1970, the switch to 6138B took place in 1972, and production lasted until 1979.