MLA
Chambers, R. T. Letter from R. T. Chambers to His Mother - October 3, 1862. 1862. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 7, Folder 41. https://digital.library.emory.edu/purl/000000011h-cor.
APA, 6th edition
Chambers, R. T. (1862). Letter from R. T. Chambers to his mother - October 3, 1862 (Box 7, Folder 41) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/000000011h-cor.
Chicago
Chambers, R. T. “Letter from R. T. Chambers to His Mother - October 3, 1862.” Letters (correspondence). Civil War Collection, 1862. Box 7, Folder 41. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/000000011h-cor.