MLA

Pound, Ezra, 1885-1972. MacLeish, Archibald. Box 11, folder 259. 1962, n.d.. https://collections.library.yale.edu/catalog/17180218.

APA, 6th edition

Pound, Ezra, 1885-1972. (["Box 11, folder 259"])(1962, n.d.)[MacLeish, Archibald]. https://collections.library.yale.edu/catalog/17180218.