MLA

Stieglitz, Alfred, 1864-1946. Letters. Box 62, folder 1426. 1919 Sep. https://collections.library.yale.edu/catalog/2039789.

APA, 6th edition

Stieglitz, Alfred, 1864-1946. (["Box 62, folder 1426"])(1919 Sep)[Letters]. https://collections.library.yale.edu/catalog/2039789.