MLA
Brobst, Nathan. Letter from Nathan Brobst to Sarah Ann Scott about the Death of Her Husband, Alexander Scott - January 31, 1865. 1865. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 6, Folder 15. https://digital.library.emory.edu/purl/730bvq842s-cor.
APA, 6th edition
Brobst, N. (1865). Letter from Nathan Brobst to Sarah Ann Scott about the death of her husband, Alexander Scott - January 31, 1865 (Box 6, Folder 15) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/730bvq842s-cor.
Chicago
Brobst, Nathan. “Letter from Nathan Brobst to Sarah Ann Scott about the Death of Her Husband, Alexander Scott - January 31, 1865.” Letters (correspondence). Civil War Collection, 1865. Box 6, Folder 15. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/730bvq842s-cor.