MLA

Wat, Aleksander, 1900-1967. Tuwim, Julian. Box 6, folder 225. 1947, undated. https://collections.library.yale.edu/catalog/2053961.

APA, 6th edition

Wat, Aleksander, 1900-1967. (["Box 6, folder 225"])(1947, undated)[Tuwim, Julian]. https://collections.library.yale.edu/catalog/2053961.