MLA
Copenhaver, Thomas. Letter from Thomas Copenhaver (Salem, Virginia) to "Dear Father" - April 20, 1861. 1861. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 2, Folder 36. https://digital.library.emory.edu/purl/508hdr7tp2-cor.
APA, 6th edition
Copenhaver, T. (1861). Letter from Thomas Copenhaver (Salem, Virginia) to "Dear father" - April 20, 1861 (Box 2, Folder 36) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/508hdr7tp2-cor.
Chicago
Copenhaver, Thomas. “Letter from Thomas Copenhaver (Salem, Virginia) to ‘Dear Father’ - April 20, 1861.” Letters (correspondence). Civil War Collection, 1861. Box 2, Folder 36. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/508hdr7tp2-cor.