MLA

Miłosz, Czesław, 1911-2004. Iwaszkiewicz, Jarosław. Box 143, folder 2260. undated. https://collections.library.yale.edu/catalog/10050465.

APA, 6th edition

Miłosz, Czesław, 1911-2004. (["Box 143, folder 2260"])(undated)[Iwaszkiewicz, Jarosław]. https://collections.library.yale.edu/catalog/10050465.