MLA
Slaughter, Montgomery. Letter from the Mayor of Fredericksburg, Montgomery Slaughter to Colonel Daniel Ruggles - May 27, 1861. 1861. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 6, Folder 6. https://digital.library.emory.edu/purl/948sbcc35v-cor.
APA, 6th edition
Slaughter, M. (1861). Letter from the mayor of Fredericksburg, Montgomery Slaughter to Colonel Daniel Ruggles - May 27, 1861 (Box 6, Folder 6) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/948sbcc35v-cor.
Chicago
Slaughter, Montgomery. “Letter from the Mayor of Fredericksburg, Montgomery Slaughter to Colonel Daniel Ruggles - May 27, 1861.” Letters (correspondence). Civil War Collection, 1861. Box 6, Folder 6. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/948sbcc35v-cor.