MLA

Henri, Robert, 1865-1929. Lawson, Ernest. Box 6, folder 142. 1906-[14?], n.d.. https://collections.library.yale.edu/catalog/17029568.

APA, 6th edition

Henri, Robert, 1865-1929. (["Box 6, folder 142"])(1906-[14?], n.d.)[Lawson, Ernest]. https://collections.library.yale.edu/catalog/17029568.