Veterans Administration Master Index, 1917 - 9/16/1940. NAI 76193916. Record Group 15: Records of the Department of Veterans Affairs, 1773 - 2007. National Archives at St. Louis, St. Louis, Missouri, U.S.A.
This collection contains a card index created by the Veterans Bureau (Veterans Administration) for each veteran who had insurance applications, conversions, veterans who had filed claims with their agency, and veterans who were paid a WWI bonus.
You can learn more about this collection at the FamilySearch website.