MLA

Cullen, Countee, 1903-1946. Braithwaite, Fiona. Box 1, folder 6. 1926 Apr 22. https://collections.library.yale.edu/catalog/16731053.

APA, 6th edition

Cullen, Countee, 1903-1946. (["Box 1, folder 6"])(1926 Apr 22)[Braithwaite, Fiona]. https://collections.library.yale.edu/catalog/16731053.