MLA
Letter from an Unknown Soldier (Catlettsburg, Kentucky) to "Dear Friend". Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 6, Folder 66. https://digital.library.emory.edu/purl/1676t1g26n-cor.
APA, 6th edition
Letter from an unknown soldier (Catlettsburg, Kentucky) to "Dear Friend" (Box 6, Folder 66). [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/1676t1g26n-cor.
Chicago
“Letter from an Unknown Soldier (Catlettsburg, Kentucky) to ‘Dear Friend.’” Letters (correspondence). Civil War Collection, n.d. Box 6, Folder 66. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/1676t1g26n-cor.