MLA

McKay, Claude, 1890-1948. Cooper, Rachel McKay. Box 2, folder 39. 1927-29. https://collections.library.yale.edu/catalog/32180818.

APA, 6th edition

McKay, Claude, 1890-1948. (["Box 2, folder 39"])(1927-29)[Cooper, Rachel McKay]. https://collections.library.yale.edu/catalog/32180818.