MLA
Armistead, Mary Keen. Letter from Mary Keen Armistead (Petersburg, Virginia) to "My Very Dear Sister", Fannie Keen (Richmond, Virginia) - December, 1864. 1864. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 4, Folder 25. https://digital.library.emory.edu/purl/709w0vt5cw-cor.
APA, 6th edition
Armistead, M. K. (1864). Letter from Mary Keen Armistead (Petersburg, Virginia) to "My very dear sister", Fannie Keen (Richmond, Virginia) - December, 1864 (Box 4, Folder 25) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/709w0vt5cw-cor.
Chicago
Armistead, Mary Keen. “Letter from Mary Keen Armistead (Petersburg, Virginia) to ‘My Very Dear Sister’, Fannie Keen (Richmond, Virginia) - December, 1864.” Letters (correspondence). Civil War Collection, 1864. Box 4, Folder 25. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/709w0vt5cw-cor.