MLA

Stieglitz, Alfred, 1864-1946. "P" general. Box 38, folder 901. 1889-1946, n.d.. https://collections.library.yale.edu/catalog/10700783.

APA, 6th edition

Stieglitz, Alfred, 1864-1946. (["Box 38, folder 901"])(1889-1946, n.d.)["P" general]. https://collections.library.yale.edu/catalog/10700783.