MLA
Clark, Walter H. Letter from Walter H. Clark to Milton Clark - November 28, [Unknown Year]. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 2, Folder 21. https://digital.library.emory.edu/purl/907tmpg5dv-cor.
APA, 6th edition
Clark, W. H. Letter from Walter H. Clark to Milton Clark - November 28, [Unknown Year] (Box 2, Folder 21) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/907tmpg5dv-cor.
Chicago
Clark, Walter H. “Letter from Walter H. Clark to Milton Clark - November 28, [Unknown Year].” Letters (correspondence). Civil War Collection, n.d. Box 2, Folder 21. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/907tmpg5dv-cor.