MLA
Riboud, David. Letter from David Riboud to His Grandmother, Vivian May Chase (Philadelphia, Pennsylvania) - September 14, 1974. 1974. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 1, Folder 8. https://digital.library.emory.edu/purl/904dncjtvr-cor.
APA, 6th edition
Riboud, D. (1974). Letter from David Riboud to his grandmother, Vivian May Chase (Philadelphia, Pennsylvania) - September 14, 1974 (Box 1, Folder 8) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/904dncjtvr-cor.
Chicago
Riboud, David. “Letter from David Riboud to His Grandmother, Vivian May Chase (Philadelphia, Pennsylvania) - September 14, 1974.” Letters (correspondence). Barbara Chase-Riboud Papers, 1974. Box 1, Folder 8. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/904dncjtvr-cor.