MLA
Bell, U. S. Letter from U. S. Bell to Alexander W. Wylly - April 4, 1864. 1864. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 7, Folder 46. https://digital.library.emory.edu/purl/228ncjszkw-cor.
APA, 6th edition
Bell, U. S. (1864). Letter from U. S. Bell to Alexander W. Wylly - April 4, 1864 (Box 7, Folder 46) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/228ncjszkw-cor.
Chicago
Bell, U. S. “Letter from U. S. Bell to Alexander W. Wylly - April 4, 1864.” Letters (correspondence). Civil War Collection, 1864. Box 7, Folder 46. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/228ncjszkw-cor.