MLA

Koch, Frederick R. (Frederick Robinson). Offenbach, Pepita. Box 47, folder 941. [1875 Jul 8]. https://collections.library.yale.edu/catalog/12214804.

APA, 6th edition

Koch, Frederick R. (Frederick Robinson). (["Box 47, folder 941"])([1875 Jul 8])[Offenbach, Pepita]. https://collections.library.yale.edu/catalog/12214804.