MLA

Serge, Victor, 1890-1947. Martinet, Marcel. Box 1, folder 53. 1939-40. https://collections.library.yale.edu/catalog/16495438.

APA, 6th edition

Serge, Victor, 1890-1947. (["Box 1, folder 53"])(1939-40)[Martinet, Marcel]. https://collections.library.yale.edu/catalog/16495438.