MLA

Koch, Frederick R. (Frederick Robinson). Raverat, Gwen. Box 57, folder 1296. [1931] Jul 12. https://collections.library.yale.edu/catalog/17341485.

APA, 6th edition

Koch, Frederick R. (Frederick Robinson). (["Box 57, folder 1296"])([1931] Jul 12)[Raverat, Gwen]. https://collections.library.yale.edu/catalog/17341485.