MLA
Clay [unknown last name]. Letter from Clay to Lizzie (Miss M. E. Beard) - September 22nd, 1864. September 22, 1864. Civil War Collection. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/423612jn7p-cor.
APA, 6th edition
Clay [unknown last name]. (September 22, 1864)[Letter from Clay to Lizzie (Miss M. E. Beard) - September 22nd, 1864]. Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/423612jn7p-cor.
Chicago
Clay [unknown last name], Letter from Clay to Lizzie (Miss M. E. Beard) - September 22nd, 1864, September 22, 1864, Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/423612jn7p-cor.