Service number
From 1948, under the Selective Service Act, all American men aged 18 through 25 were required to register with a local draft board. In case of war, the able-bodied ones among them could be drafted to serve in the military. The law required the men to carry their draft cards with them at all times. These were small cards bearing the registrant's identifying information, the date and place of registration, and a unique Selective Service number.