MLA

Schwartz, Delmore, 1913-1966. Cooper, Pat. Box 1, folder 24. 1953, undated. https://collections.library.yale.edu/catalog/17078923.

APA, 6th edition

Schwartz, Delmore, 1913-1966. (["Box 1, folder 24"])(1953, undated)[Cooper, Pat]. https://collections.library.yale.edu/catalog/17078923.