MLA

Cullen, Countee, 1903-1946. Gross, ---. Box 1, folder 17. 1945 May 8. https://collections.library.yale.edu/catalog/16731063.

APA, 6th edition

Cullen, Countee, 1903-1946. (["Box 1, folder 17"])(1945 May 8)[Gross, ---]. https://collections.library.yale.edu/catalog/16731063.