MLA

Sartre, Jean-Paul, 1905-1980. Kozakievitch-Bost, Olga. Box 3, folder 46. 1973 May 9. https://collections.library.yale.edu/catalog/31300403.

APA, 6th edition

Sartre, Jean-Paul, 1905-1980. (["Box 3, folder 46"])(1973 May 9)[Kozakievitch-Bost, Olga]. https://collections.library.yale.edu/catalog/31300403.