MLA
Clark, Milton. Letter from Milton Clark to "Dear Brothers" - May 14, 1866. 1866. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 2, Folder 21. https://digital.library.emory.edu/purl/474cnp5jqf-cor.
APA, 6th edition
Clark, M. (1866). Letter from Milton Clark to "Dear Brothers" - May 14, 1866 (Box 2, Folder 21) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/474cnp5jqf-cor.
Chicago
Clark, Milton. “Letter from Milton Clark to ‘Dear Brothers’ - May 14, 1866.” Letters (correspondence). Civil War Collection, 1866. Box 2, Folder 21. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/474cnp5jqf-cor.