MLA
Chase-Riboud, Barbara. Postcard from Barbara Chase-Riboud to Her Grandfather - May 20, 1966. 1966. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 1, Folder 4. https://digital.library.emory.edu/purl/095j9kd5xm-cor.
APA, 6th edition
Chase-Riboud, B. (1966). Postcard from Barbara Chase-Riboud to her grandfather - May 20, 1966 (Box 1, Folder 4) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/095j9kd5xm-cor.
Chicago
Chase-Riboud, Barbara. “Postcard from Barbara Chase-Riboud to Her Grandfather - May 20, 1966.” Letters (correspondence). Barbara Chase-Riboud Papers, 1966. Box 1, Folder 4. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/095j9kd5xm-cor.