MLA

Matthiessen, F. O. (Francis Otto), 1902-1950. Untermeyer, Louis. Box 4. 1949 April 2. https://collections.library.yale.edu/catalog/32239990.

APA, 6th edition

Matthiessen, F. O. (Francis Otto), 1902-1950. (["Box 4"])(1949 April 2)[Untermeyer, Louis]. https://collections.library.yale.edu/catalog/32239990.