MLA

Koch, Frederick R. (Frederick Robinson). Bennett, Arnold. Box 51, folder 1080. 1920 May 21. https://collections.library.yale.edu/catalog/2065829.

APA, 6th edition

Koch, Frederick R. (Frederick Robinson). (["Box 51, folder 1080"])(1920 May 21)[Bennett, Arnold]. https://collections.library.yale.edu/catalog/2065829.