MLA

Stein, Gertrude, 1874-1946. Beaton, Cecil. Box 136, folder 3087. 1946 Aug 1. https://collections.library.yale.edu/catalog/10189455.

APA, 6th edition

Stein, Gertrude, 1874-1946. (["Box 136, folder 3087"])(1946 Aug 1)[Beaton, Cecil]. https://collections.library.yale.edu/catalog/10189455.