The Seiko 2906-0239 watch movement is a basic handwound movement with 18 jewels and 117 parts. It is Hallmark of Geneva certified. Developed beginning in 1952 and presented in 1955 for the brand's bicentenary celebrations, the Calibre 1003 holds the distinction of being the world's smallest mechanical manual-winding movement with a diameter of 9-lignes or 21.05 mm and a thickness of 1.64 mm. It consists of 120 parts and has been used to equip three round watches that set a new record as the world's thinnest watches, measuring just 4.54 mm thick. The movement was primarily used in watches produced around 1960.