MLA

Schletter, Annie, 1858-1944. Cooper, Gladys. Box 1, folder 9. 1917 Sep 14. https://collections.library.yale.edu/catalog/17453741.

APA, 6th edition

Schletter, Annie, 1858-1944. (["Box 1, folder 9"])(1917 Sep 14)[Cooper, Gladys]. https://collections.library.yale.edu/catalog/17453741.