MLA

Bullitt, William C. (William Christian), 1891-1967. Polo. Box 180, folder 11 - 12. undated. https://collections.library.yale.edu/catalog/10687864.

APA, 6th edition

Bullitt, William C. (William Christian), 1891-1967. (["Box 180, folder 11 - 12"])(undated)[Polo]. https://collections.library.yale.edu/catalog/10687864.