MLA

Steinberg, Saul, 1914-1999. Cheyenne, Wyoming. Box 420, folder 5660. undated. https://collections.library.yale.edu/catalog/16849952.

APA, 6th edition

Steinberg, Saul, 1914-1999. (["Box 420, folder 5660"])(undated)[Cheyenne, Wyoming]. https://collections.library.yale.edu/catalog/16849952.