MLA

Stein, Gertrude, 1874-1946. Tschoukine, Serge. Box 128, folder 2786. 1912 Nov 30. https://collections.library.yale.edu/catalog/17105775.

APA, 6th edition

Stein, Gertrude, 1874-1946. (["Box 128, folder 2786"])(1912 Nov 30)[Tschoukine, Serge]. https://collections.library.yale.edu/catalog/17105775.