MLA

Schletter, Annie, 1858-1944. Castle, Agnes Egerton. Box 1, folder 5. 1907 Mar 28. https://collections.library.yale.edu/catalog/17453738.

APA, 6th edition

Schletter, Annie, 1858-1944. (["Box 1, folder 5"])(1907 Mar 28)[Castle, Agnes Egerton]. https://collections.library.yale.edu/catalog/17453738.