Enlistment
Early digital records: The US Army was unique in using punch cards to register new enlistees. This in turn made it easy to create an electronic database of WWII enlistments in 2002 from those punch cards. You can search this resource to find, for example, someone’s place and date of enlistment, their Army serial number, and their birth year.
A WWII Army enlistment punch card. Featured in National Archives blog “The World War II Army Enlistment Records File and Access to Archival Databases,” 2006.