MLA
Clay [unknown last name]. Letter from Clay to Lizzie (Miss M. E. Beard) - April 8th, 1864. April 8, 1864. Civil War Collection. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/839k3j9mdg-cor.
APA, 6th edition
Clay [unknown last name]. (April 8, 1864)[Letter from Clay to Lizzie (Miss M. E. Beard) - April 8th, 1864]. Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/839k3j9mdg-cor.
Chicago
Clay [unknown last name], Letter from Clay to Lizzie (Miss M. E. Beard) - April 8th, 1864, April 8, 1864, Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/839k3j9mdg-cor.