MLA

Pound, Ezra, 1885-1972. Letters. Box 60, folder 2683. 1921. https://collections.library.yale.edu/catalog/2054710.

APA, 6th edition

Pound, Ezra, 1885-1972. (["Box 60, folder 2683"])(1921)[Letters]. https://collections.library.yale.edu/catalog/2054710.