MLA

Koch, Frederick R. (Frederick Robinson). Music/Unidentified/Piano music, theme and variations. Box 346, folder 2054. n.d.. https://collections.library.yale.edu/catalog/2043682.

APA, 6th edition

Koch, Frederick R. (Frederick Robinson). (["Box 346, folder 2054"])(n.d.)[Music/Unidentified/Piano music, theme and variations]. https://collections.library.yale.edu/catalog/2043682.