MLA
Clay [unknown last name]. Letter from Clay to Lizzie (Miss M. E. Beard) - June 2nd, 1863. June 2, 1863. Civil War Collection. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/518rbnzt7r-cor.
APA, 6th edition
Clay [unknown last name]. (June 2, 1863)[Letter from Clay to Lizzie (Miss M. E. Beard) - June 2nd, 1863]. Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/518rbnzt7r-cor.
Chicago
Clay [unknown last name], Letter from Clay to Lizzie (Miss M. E. Beard) - June 2nd, 1863, June 2, 1863, Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/518rbnzt7r-cor.