The Seiko 6139 and 6138 series - like this full serviced Japanese Domestic Market (JDM)-only 1975 Seiko 6138-0030 SpeedTimer "Kakume" automatic chronograph here - were the first fully integrated automatic chronograph in the world featuring column wheel and vertical coupling mechanism. 
 
A unique feature of these sporty 6138-0030’s were square hour and minute subdials, hence its nickname “kakume,” Japanese for “square eyes.”  Unusual for the era at the time, this 6138 is rather substantial in size measuring in at 43mm in width.
 
With the 6138 movement, Seiko - as usual - set the standard followed by the Swiss watch industry 20 years later.  The same movement configuration was adopted by Rolex when it made its own in-house movement for the Daytona in 2000, 30 years after Seiko released its 6139 and 6138 calibres.  Oddly, the 6138 was the successor of the 6139 automatic chronograph movement, and produced in parallel with it until 1979.

The 6138 was technically at least equal to its Swiss counterparts, as it featured a fully integrated chronograph vice stacked modules.  There were two versions of the 6138, with the first, the 6138A, introduced in 1970 with 21 jewels.  The 6138B – like the one for sale here – was introduced subsequent, and featured 23 jewels. 
 
Both had a column wheel that regulated chrono functions, and a vertical clutch mechanism for engagement.  The 6138A/B was a two register chronograph, counting up to 30 minutes on one subdial and another 12 hours on the second dial.  All 6138 movements feature a quick-set day/date indicator and can be handwound (unlike the 6139 model), relatively unique for Seiko chronographs of this era.
 
This 6138 Kakume comes on a stainless-steel Seiko bracelet, and with nylon strap, rugged travel case, and springbar tool.