MLA
Gregg, Maxey. Letter from Colonel Maxey Gregg (Sullivan's Island) to David Flavel Jamison - January 27, 1861. 1861. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 4, Folder 7. https://digital.library.emory.edu/purl/52908kpstp-cor.
APA, 6th edition
Gregg, M. (1861). Letter from Colonel Maxey Gregg (Sullivan's Island) to David Flavel Jamison - January 27, 1861 (Box 4, Folder 7) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/52908kpstp-cor.
Chicago
Gregg, Maxey. “Letter from Colonel Maxey Gregg (Sullivan's Island) to David Flavel Jamison - January 27, 1861.” Letters (correspondence). Civil War Collection, 1861. Box 4, Folder 7. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/52908kpstp-cor.