MLA

Koch, Frederick R. (Frederick Robinson). Music/Mai. Box 321, folder 1726. n.d.. https://collections.library.yale.edu/catalog/2053110.

APA, 6th edition

Koch, Frederick R. (Frederick Robinson). (["Box 321, folder 1726"])(n.d.)[Music/Mai]. https://collections.library.yale.edu/catalog/2053110.