MLA

Sartre, Jean-Paul, 1905-1980. Elkaim-Sartre, Arlette. Box 3, folder 42. 1973 Mar 5. https://collections.library.yale.edu/catalog/16538190.

APA, 6th edition

Sartre, Jean-Paul, 1905-1980. (["Box 3, folder 42"])(1973 Mar 5)[Elkaim-Sartre, Arlette]. https://collections.library.yale.edu/catalog/16538190.