MLA

Van Thal, Herbert, 1904-1983. Lehmann, Beatrix. Box 1, folder 75. n.D.. https://collections.library.yale.edu/catalog/16545979.

APA, 6th edition

Van Thal, Herbert, 1904-1983. (["Box 1, folder 75"])(n.D.)[Lehmann, Beatrix]. https://collections.library.yale.edu/catalog/16545979.