MLA
Clapp[?], P. Letter from P. Clapp[?] (Iowa City, Iowa) to Brother and Sister - December 10, 1861[?]. 1861. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 19, Folder 12. https://digital.library.emory.edu/purl/3009w0vv8h-cor.
APA, 6th edition
Clapp[?], P. (1861). Letter from P. Clapp[?] (Iowa City, Iowa) to brother and sister - December 10, 1861[?] (Box 19, Folder 12) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/3009w0vv8h-cor.
Chicago
Clapp[?], P. “Letter from P. Clapp[?] (Iowa City, Iowa) to Brother and Sister - December 10, 1861[?].” Letters (correspondence). Civil War Collection, 1861. Box 19, Folder 12. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/3009w0vv8h-cor.