The Seiko 6139 and 6138 series - this all original serviced 1976 Seiko 6138-3002 "Jumbo" here counts itself among the latter - were the first fully integrated automatic chronograph in the world featuring column wheel and vertical coupling mechanism. The so-called "jumbo" is sizable, compared to the smaller 6139 "mini-jumbo" of nearly identical design.
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 Jumbo comes with a leather rally strap, nylon NATO strap, Pelican travel case, and springbar tool.
1976 Seiko 6138-3002 "Jumbo" Automatic Chronograph
DIAL: Original Seiko-signed dial; day and date function - with the Spanish and English day variants - at the 3 o'clock position both work correctly as designed.
CASE: Sizable original 42.5mm (w/o crown, 45mm with) x 48.5mm stainless-steel case. Matching original caseback.
CRYSTAL: Slightly domed Seiko 6138-correct crystal, no scratches or imperfections.
BAND: This Jumbo comes with a dark brown leather strap with white accent stitching; it also comes with a green and yellow nylon NATO strap, with stainless steel hardware.
MOVEMENT: Original 6128B Seiko 21-jewel automatic movement, manufactured in March 1976, which beats at 21,600 beats per hour. This watch has been serviced.
CROWN: Original unsigned stainless steel crown.