MLA

Pound, Ezra, 1885-1972. Heyman, Katherine Ruth. Box 22, folder 975. [1906]. https://collections.library.yale.edu/catalog/17122575.

APA, 6th edition

Pound, Ezra, 1885-1972. (["Box 22, folder 975"])([1906])[Heyman, Katherine Ruth]. https://collections.library.yale.edu/catalog/17122575.