MLA

Cullen, Countee, 1903-1946. Sebree, Charles. Box 2, folder 23. 1940. https://collections.library.yale.edu/catalog/16731069.

APA, 6th edition

Cullen, Countee, 1903-1946. (["Box 2, folder 23"])(1940)[Sebree, Charles]. https://collections.library.yale.edu/catalog/16731069.