The Seiko 66A is a manual-wind chronograph movement that was produced between 1937 and 1970. Over 3.5 million units of this movement were manufactured during this period. The movement featured various chrono mechanisms, and certain series may have utilized parts from different calibres, although the specific differences between the 248 and 348 calibres are not known. Additionally, the Seiko 66A movement included functions such as a 60-second chronograph, a 30/45-minute chronograph, and a sub-second display.