Seiko 5206

The Seiko caliber 5206 is an automatic movement introduced in 1970, featuring a day/date calendar with quickset functionality for both displays. It operates at 28,800 vph and uses a ball-bearing rotor, with two variants offering either 23 or 25 jewels. The movement is notable for its "spontaneous" instant day/date changeover at midnight—a feature lost when it evolved into the caliber 5216A around 1972, which adopted a more efficient calendar system but with a delayed day change. It was used in Lord Matic watches produced by Daini Seikosha between approximately 1970 and 1973.