MLA

Stieglitz, Alfred, 1864-1946. Arbus, Diane. Box 3, folder 51a. 1945 Apr 8. https://collections.library.yale.edu/catalog/10080231.

APA, 6th edition

Stieglitz, Alfred, 1864-1946. (["Box 3, folder 51a"])(1945 Apr 8)[Arbus, Diane]. https://collections.library.yale.edu/catalog/10080231.