The Seiko 2501B watch movement, produced during the period of 1949-1950, belongs to the first gents automatic-family with a ball-bearing rotor. It underwent modifications such as a change from T- to U shock device and a modified calibre, resulting in several variants. While it remains uncertain if all calibres were made in the TC variant, the known variants are listed. The signatures of UT versions were also altered to UC. In addition to its automatic winding feature, this movement also includes a sweep second hand and a date function.