MLA

Serge, Victor, 1890-1947. Rosenthal, Gérard. Box 2, folder 73. 1939 Apr 26. https://collections.library.yale.edu/catalog/17455290.

APA, 6th edition

Serge, Victor, 1890-1947. (["Box 2, folder 73"])(1939 Apr 26)[Rosenthal, Gérard]. https://collections.library.yale.edu/catalog/17455290.