MLA

Seligmann, Kurt, 1900-1962. Breton, André. Box 30. 1939-1941, undated. https://collections.library.yale.edu/catalog/11693644.

APA, 6th edition

Seligmann, Kurt, 1900-1962. (["Box 30"])(1939-1941, undated)[Breton, André]. https://collections.library.yale.edu/catalog/11693644.