MLA

Stein, Gertrude, 1874-1946. Hockaday, Ela. Box 110, folder 2250. 1937-[39?], n.d.. https://collections.library.yale.edu/catalog/10105362.

APA, 6th edition

Stein, Gertrude, 1874-1946. (["Box 110, folder 2250"])(1937-[39?], n.d.)[Hockaday, Ela]. https://collections.library.yale.edu/catalog/10105362.