MLA

Koch, Frederick R. (Frederick Robinson). Other Papers/Bérard, Christian. Drawing of Poulenc. Box 331, folder 1875. n.d.. https://collections.library.yale.edu/catalog/16601877.

APA, 6th edition

Koch, Frederick R. (Frederick Robinson). (["Box 331, folder 1875"])(n.d.)[Other Papers/Bérard, Christian. Drawing of Poulenc]. https://collections.library.yale.edu/catalog/16601877.