MLA

Butts, Mary, 1890-1937. Wescott, Glenway. Box 2, folder 43. n.d.. https://collections.library.yale.edu/catalog/16947642.

APA, 6th edition

Butts, Mary, 1890-1937. (["Box 2, folder 43"])(n.d.)[Wescott, Glenway]. https://collections.library.yale.edu/catalog/16947642.