MLA

Stieglitz, Alfred, 1864-1946. Letters. Box 63, folder 1442. 1927 Apr. https://collections.library.yale.edu/catalog/2039847.

APA, 6th edition

Stieglitz, Alfred, 1864-1946. (["Box 63, folder 1442"])(1927 Apr)[Letters]. https://collections.library.yale.edu/catalog/2039847.