MLA

Koch, Frederick R. (Frederick Robinson). Daudet, Alphonse. Box 20, folder 369. 1880 Jun 21. https://collections.library.yale.edu/catalog/2007270.

APA, 6th edition

Koch, Frederick R. (Frederick Robinson). (["Box 20, folder 369"])(1880 Jun 21)[Daudet, Alphonse]. https://collections.library.yale.edu/catalog/2007270.