MLA
Reynolds, Robert D. Letter from Robert D. Reynolds (Montgomery and Mobile Railroad) to "Dear Cousin." Reynolds Served in Company F of the 15th Alabama Cavalry - August 21, 1863. 1863. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 5, Folder 61. https://digital.library.emory.edu/purl/238w9ghz45-cor.
APA, 6th edition
Reynolds, R. D. (1863). Letter from Robert D. Reynolds (Montgomery and Mobile Railroad) to "Dear Cousin." Reynolds served in Company F of the 15th Alabama Cavalry - August 21, 1863 (Box 5, Folder 61) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/238w9ghz45-cor.
Chicago
Reynolds, Robert D. “Letter from Robert D. Reynolds (Montgomery and Mobile Railroad) to ‘Dear Cousin." Reynolds Served in Company F of the 15th Alabama Cavalry - August 21, 1863.” Letters (correspondence). Civil War Collection, 1863. Box 5, Folder 61. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/238w9ghz45-cor.