MLA

Serge, Victor, 1890-1947. Balabanoff, Angelica. Box 1, folder 6. 1941 Oct 9. https://collections.library.yale.edu/catalog/16495420.

APA, 6th edition

Serge, Victor, 1890-1947. (["Box 1, folder 6"])(1941 Oct 9)[Balabanoff, Angelica]. https://collections.library.yale.edu/catalog/16495420.