MLA
Waller, William. Letters from William Waller (Lynchburg, Kentucky) to "My Dear Sir," Regarding the Death of His Son (Includes a Lock of Hair) - June 20, 1865. 1865. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 9, Folder 42. https://digital.library.emory.edu/purl/286nzs7j5x-cor.
APA, 6th edition
Waller, W. (1865). Letters from William Waller (Lynchburg, Kentucky) to "My dear sir," regarding the death of his son (includes a lock of hair) - June 20, 1865 (Box 9, Folder 42) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/286nzs7j5x-cor.
Chicago
Waller, William. “Letters from William Waller (Lynchburg, Kentucky) to ‘My Dear Sir," Regarding the Death of His Son (Includes a Lock of Hair) - June 20, 1865.” Letters (correspondence). Civil War Collection, 1865. Box 9, Folder 42. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/286nzs7j5x-cor.