MLA

Koch, Frederick R. (Frederick Robinson). Music/Duets for Children. Box 337, folder 1956. [1940]. https://collections.library.yale.edu/catalog/2003061.

APA, 6th edition

Koch, Frederick R. (Frederick Robinson). (["Box 337, folder 1956"])([1940])[Music/Duets for Children]. https://collections.library.yale.edu/catalog/2003061.