serial numbers
Seiko manufactures a variety of watches, and each model of watch has its own unique serial number. The 401-HEAD part of Seiko serial numbers refer to the case style of the watch, which is defined by four digits, followed by a dash, followed by a maximum of three letters. For example, 4206-CAR is the case style for a Seiko 5 Sports watch. The two digits refer to the size and design of the case, while the letters indicate the type of material used. The letters may be omitted in some serial numbers.