MLA

Stieglitz, Alfred, 1864-1946. Schubart, Selma Stieglitz. Box 1, folder 11. 1957 Jun 27. https://collections.library.yale.edu/catalog/32500313.

APA, 6th edition

Stieglitz, Alfred, 1864-1946. (["Box 1, folder 11"])(1957 Jun 27)[Schubart, Selma Stieglitz]. https://collections.library.yale.edu/catalog/32500313.