MLA

Stein, Gertrude, 1874-1946. Stein, Leo. Box 172, folder 4354. 1915, n.d.. https://collections.library.yale.edu/catalog/15604337.

APA, 6th edition

Stein, Gertrude, 1874-1946. (["Box 172, folder 4354"])(1915, n.d.)[Stein, Leo]. https://collections.library.yale.edu/catalog/15604337.