MLA

Serge, Victor, 1890-1947. Trotsky, Leon. Box 2, folder 81. 1936. https://collections.library.yale.edu/catalog/17455291.

APA, 6th edition

Serge, Victor, 1890-1947. (["Box 2, folder 81"])(1936)[Trotsky, Leon]. https://collections.library.yale.edu/catalog/17455291.