MLA

Stieglitz, Alfred, 1864-1946. Troubetzkoy, Pierre. Box 50, folder 1181. 1921, n.d.. https://collections.library.yale.edu/catalog/17414991.

APA, 6th edition

Stieglitz, Alfred, 1864-1946. (["Box 50, folder 1181"])(1921, n.d.)[Troubetzkoy, Pierre]. https://collections.library.yale.edu/catalog/17414991.