MLA

Cullen, Countee, 1903-1946. Byrd, Sydonia. Box 1, folder 7. 1927 Jan 11. https://collections.library.yale.edu/catalog/16731054.

APA, 6th edition

Cullen, Countee, 1903-1946. (["Box 1, folder 7"])(1927 Jan 11)[Byrd, Sydonia]. https://collections.library.yale.edu/catalog/16731054.