MLA
Mackintosh, Mary. Letter from Mary Mackintosh to [Unknown] - April 13, 1874[?] (Photocopy). 1874. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 4, Folder 50. https://digital.library.emory.edu/purl/351hhmgrqb-cor.
APA, 6th edition
Mackintosh, M. (1874). Letter from Mary Mackintosh to [unknown] - April 13, 1874[?] (photocopy) (Box 4, Folder 50) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/351hhmgrqb-cor.
Chicago
Mackintosh, Mary. “Letter from Mary Mackintosh to [Unknown] - April 13, 1874[?] (Photocopy).” Letters (correspondence). Civil War Collection, 1874. Box 4, Folder 50. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/351hhmgrqb-cor.