MLA
Chase-Riboud, Barbara. Letter from Barbara Chase-Riboud (Geneva, Switzerland) to Her Mother, Vivian May Chase (Philadelphia, Pennsylvania) - April 19, [No Year Provided]. 1968. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 1, Folder 10. https://digital.library.emory.edu/purl/221tdz09h1-cor.
APA, 6th edition
Chase-Riboud, B. (1968). Letter from Barbara Chase-Riboud (Geneva, Switzerland) to her mother, Vivian May Chase (Philadelphia, Pennsylvania) - April 19, [No Year Provided] (Box 1, Folder 10) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/221tdz09h1-cor.
Chicago
Chase-Riboud, Barbara. “Letter from Barbara Chase-Riboud (Geneva, Switzerland) to Her Mother, Vivian May Chase (Philadelphia, Pennsylvania) - April 19, [No Year Provided].” Letters (correspondence). Barbara Chase-Riboud Papers, 1968. Box 1, Folder 10. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/221tdz09h1-cor.