MLA

Stieglitz, Alfred, 1864-1946. MacMahon, Arthur. Box 201, folder 3470. 1914-17, n.d.. https://collections.library.yale.edu/catalog/17105687.

APA, 6th edition

Stieglitz, Alfred, 1864-1946. (["Box 201, folder 3470"])(1914-17, n.d.)[MacMahon, Arthur]. https://collections.library.yale.edu/catalog/17105687.