MLA
Clay [unknown last name]. Letter from Clay to Lizzie (Miss M. E. Beard) - January 27th, 1865. January 27, 1865. Civil War Collection. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/3483j9kf4q-cor.
APA, 6th edition
Clay [unknown last name]. (January 27, 1865)[Letter from Clay to Lizzie (Miss M. E. Beard) - January 27th, 1865]. Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/3483j9kf4q-cor.
Chicago
Clay [unknown last name], Letter from Clay to Lizzie (Miss M. E. Beard) - January 27th, 1865, January 27, 1865, Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/3483j9kf4q-cor.