A military serial number, also referred to as a service number, is a unique number assigned to all military personnel. The U.S. military used this system as a means of identification of service members between 1918 and 1974, when social security numbers replaced them.
Each branch of the military had a system for assigning serial numbers, and a soldier’s serial number provided information, such as whether the member had enlisted or been drafted and into which corps they were inducted. Serial numbers originally contained numeric characters. The Navy adopted the use of letter prefixes and suffixes to provide additional information in 1920. For example, the serial numbers of female enlisted personnel contained the suffix “W.” Other branches of the military also adopted this practice, which continued until 1969.