Seiko's 51 Stream was a family of midrange automatic watch movements from Seiko produced in the late 1960's into the early 1970's. It is notable for the quickset date pusher located inside the crown and this function was the namesake for the Seiko “Presmatic” line. It was produced by Daini Seikosha.

Three 51 Stream movements were introduced in 1967: The 33 jewel 19,800 A/h 5106 found in the domestic “Seikomatic-P”, and two “Seiko New Fives”, the basic 23-jewel 5126 and the “DX” (deluxe) 5139 with 27 jewels. In 1969, a Hi-Beat (28,800 A/h) “Presmatic” was introduced with a new 5146 movement sporting 27 or 30 jewels, just as the 5139 was phased out. The movement stream began disappearing just one year later, with the 5106 and 5126 ending production in 1970 and the last surviving member, the 5146, gone in 1971.

The 51 Stream is somewhat similar in appearance to the Daini-produced 52 Stream, introduced in 1969 as the ultimate mechanical movement from that company. It is a fairly advanced movement, with hacking seconds, hand winding, a “tadpole”-style fine adjuster, and quickset date (though the day is not quickset). All 33 jewels are functional, with 6 being cap jewels and jewels on both sides of the barrel arbor. It lacks the bilingual day wheel of other contemporary Seiko movements.

• Seiko - 51 Presmatic & Seikomatic-P family