MLA

Stieglitz, Alfred, 1864-1946. Pollitzer, Anita. Box 39, folder 936. 1915-45, n.d.. https://collections.library.yale.edu/catalog/17105646.

APA, 6th edition

Stieglitz, Alfred, 1864-1946. (["Box 39, folder 936"])(1915-45, n.d.)[Pollitzer, Anita]. https://collections.library.yale.edu/catalog/17105646.