MLA
Clark, Milton. Letter from Milton Clark to Walter H. Clark - August 22, 1864. 1864. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 2, Folder 21. https://digital.library.emory.edu/purl/631cjsxmsk-cor.
APA, 6th edition
Clark, M. (1864). Letter from Milton Clark to Walter H. Clark - August 22, 1864 (Box 2, Folder 21) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/631cjsxmsk-cor.
Chicago
Clark, Milton. “Letter from Milton Clark to Walter H. Clark - August 22, 1864.” Letters (correspondence). Civil War Collection, 1864. Box 2, Folder 21. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/631cjsxmsk-cor.