MLA
Mahoney, E. Letter from E. Mahoney (Nassau, New Providence) to Mrs. William F. Black - July 20, 1863. 1863. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 4, Folder 55. https://digital.library.emory.edu/purl/81937pvndx-cor.
APA, 6th edition
Mahoney, E. (1863). Letter from E. Mahoney (Nassau, New Providence) to Mrs. William F. Black - July 20, 1863 (Box 4, Folder 55) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/81937pvndx-cor.
Chicago
Mahoney, E. “Letter from E. Mahoney (Nassau, New Providence) to Mrs. William F. Black - July 20, 1863.” Letters (correspondence). Civil War Collection, 1863. Box 4, Folder 55. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/81937pvndx-cor.