MLA

Knight, Eric, 1897-1943. Knight, Frederick. Box 15, folder 341. n.d.. https://collections.library.yale.edu/catalog/2014313.

APA, 6th edition

Knight, Eric, 1897-1943. (["Box 15, folder 341"])(n.d.)[Knight, Frederick]. https://collections.library.yale.edu/catalog/2014313.