MLA

Koch, Frederick R. (Frederick Robinson). Correspondence/Richter, Hans. Box 336, folder 1940. 1876 Jun 13. https://collections.library.yale.edu/catalog/10051209.

APA, 6th edition

Koch, Frederick R. (Frederick Robinson). (["Box 336, folder 1940"])(1876 Jun 13)[Correspondence/Richter, Hans]. https://collections.library.yale.edu/catalog/10051209.