MLA
Copenhaver, T. P. Letter from T. P. Copenhaver (Drummond, Montana) to "Sister Ella" - July 28, 1900. 1900. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Folder 35. https://digital.library.emory.edu/purl/641mgqnm91-cor.
APA, 6th edition
Copenhaver, T. P. (1900). Letter from T. P. Copenhaver (Drummond, Montana) to "Sister Ella" - July 28, 1900 (Folder 35) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/641mgqnm91-cor.
Chicago
Copenhaver, T. P. “Letter from T. P. Copenhaver (Drummond, Montana) to ‘Sister Ella’ - July 28, 1900.” Letters (correspondence). Civil War Collection, 1900. Folder 35. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/641mgqnm91-cor.