MLA

Pound, Ezra, 1885-1972. Hudson, William Henry. Box 23, folder 1010. [1920]. https://collections.library.yale.edu/catalog/31964862.

APA, 6th edition

Pound, Ezra, 1885-1972. (["Box 23, folder 1010"])([1920])[Hudson, William Henry]. https://collections.library.yale.edu/catalog/31964862.