MLA

Schletter, Annie, 1858-1944. Clark, E. Holman. Box 1, folder 7. n.d.. https://collections.library.yale.edu/catalog/17453739.

APA, 6th edition

Schletter, Annie, 1858-1944. (["Box 1, folder 7"])(n.d.)[Clark, E. Holman]. https://collections.library.yale.edu/catalog/17453739.