The Seiko 66A watch movement, produced from 1916 to 1927, is a high-quality open face calibre, model 5. It boasts a bimetallic screw balance and a Breguet hairspring. Noteworthy is its ingenious design feature where one simple part replaces the setting-lever spring, yoke, and yoke spring. This American-made movement includes functions such as stem wind and set, as well as a sub second display.