MLA

Gascoyne, David, 1916-2001. Lowell, Robert. Box 9, folder 121. 1974. https://collections.library.yale.edu/catalog/2055481.

APA, 6th edition

Gascoyne, David, 1916-2001. (["Box 9, folder 121"])(1974)[Lowell, Robert]. https://collections.library.yale.edu/catalog/2055481.