MLA
William. Letter from William (St. Louis, Missouri) to His Sister, Fannie, Regarding Their Father - February 25, 1920. 1920. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 9, Folder 49. https://digital.library.emory.edu/purl/556931zdtd-cor.
APA, 6th edition
W. (1920). Letter from William (St. Louis, Missouri) to his sister, Fannie, regarding their father - February 25, 1920 (Box 9, Folder 49) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/556931zdtd-cor.
Chicago
William. “Letter from William (St. Louis, Missouri) to His Sister, Fannie, Regarding Their Father - February 25, 1920.” Letters (correspondence). Civil War Collection, 1920. Box 9, Folder 49. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/556931zdtd-cor.