MLA

Miłosz, Czesław, 1911-2004. Sandburg, Carl. Box 140, folder 2221. undated. https://collections.library.yale.edu/catalog/10090777.

APA, 6th edition

Miłosz, Czesław, 1911-2004. (["Box 140, folder 2221"])(undated)[Sandburg, Carl]. https://collections.library.yale.edu/catalog/10090777.