MLA

Purdy, James, 1914-2009. Williams, William Carlos. Box 9, folder 219. 1956-61. https://collections.library.yale.edu/catalog/17197922.

APA, 6th edition

Purdy, James, 1914-2009. (["Box 9, folder 219"])(1956-61)[Williams, William Carlos]. https://collections.library.yale.edu/catalog/17197922.