MLA

Pound, Ezra, 1885-1972. Villa-Lobos, Heitor. Box 23, folder 518. n.d.. https://collections.library.yale.edu/catalog/32308891.

APA, 6th edition

Pound, Ezra, 1885-1972. (["Box 23, folder 518"])(n.d.)[Villa-Lobos, Heitor]. https://collections.library.yale.edu/catalog/32308891.