MLA

McKay, Claude, 1890-1948. Boardman, Helen. Box 1, folder 12. 1937, n.d.. https://collections.library.yale.edu/catalog/17022640.

APA, 6th edition

McKay, Claude, 1890-1948. (["Box 1, folder 12"])(1937, n.d.)[Boardman, Helen]. https://collections.library.yale.edu/catalog/17022640.