MLA

Wolff, Kurt, 1887-1963. Klee, Paul. Box 5, folder 176. 1919-20. https://collections.library.yale.edu/catalog/2021327.

APA, 6th edition

Wolff, Kurt, 1887-1963. (["Box 5, folder 176"])(1919-20)[Klee, Paul]. https://collections.library.yale.edu/catalog/2021327.