MLA
Letter from [Unknown] to "Dear Chas" - February 17, 1863. 1863. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 19, Folder 12. https://digital.library.emory.edu/purl/8773tx9717-cor.
APA, 6th edition
Letter from [unknown] to "Dear Chas" - February 17, 1863 (Box 19, Folder 12). (1863). [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/8773tx9717-cor.
Chicago
“Letter from [Unknown] to ‘Dear Chas’ - February 17, 1863.” Letters (correspondence). Civil War Collection, 1863. Box 19, Folder 12. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/8773tx9717-cor.