MLA

Stieglitz, Alfred, 1864-1946. Letters. Box 224, folder 3995-4006. 1968. https://collections.library.yale.edu/catalog/10575720.

APA, 6th edition

Stieglitz, Alfred, 1864-1946. (["Box 224, folder 3995-4006"])(1968)[Letters]. https://collections.library.yale.edu/catalog/10575720.