MLA
Downing, J. H. Letter from J. H. Downing to Keith M. Read Regarding the Navy Confederate Cipher - October 9, 1932. 1932. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 2, Folder 31. https://digital.library.emory.edu/purl/409j3txb60-cor.
APA, 6th edition
Downing, J. H. (1932). Letter from J. H. Downing to Keith M. Read regarding the Navy Confederate cipher - October 9, 1932 (Box 2, Folder 31) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/409j3txb60-cor.
Chicago
Downing, J. H. “Letter from J. H. Downing to Keith M. Read Regarding the Navy Confederate Cipher - October 9, 1932.” Letters (correspondence). Civil War Collection, 1932. Box 2, Folder 31. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/409j3txb60-cor.