MLA

Koch, Frederick R. (Frederick Robinson). Unidentified. Box 57, folder 1262. 1950 May. https://collections.library.yale.edu/catalog/16601866.

APA, 6th edition

Koch, Frederick R. (Frederick Robinson). (["Box 57, folder 1262"])(1950 May)[Unidentified]. https://collections.library.yale.edu/catalog/16601866.