MLA
Barr, Wolcott S. Letter from Wolcott S. Barr to His Mother, Elmira Barr. Wolcott S. Barr Served in Company I of the New York 80th Volunteer Infantry Regiment. 1863. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 19, Folder 3. https://digital.library.emory.edu/purl/757np5hrhg-cor.
APA, 6th edition
Barr, W. S. (1863). Letter from Wolcott S. Barr to his mother, Elmira Barr. Wolcott S. Barr served in Company I of the New York 80th Volunteer Infantry Regiment (Box 19, Folder 3) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/757np5hrhg-cor.
Chicago
Barr, Wolcott S. “Letter from Wolcott S. Barr to His Mother, Elmira Barr. Wolcott S. Barr Served in Company I of the New York 80th Volunteer Infantry Regiment.” Letters (correspondence). Civil War Collection, 1863. Box 19, Folder 3. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/757np5hrhg-cor.