MLA

Longstreet, Stephen, 1907-2002. Skin artist. Box 4, folder 21. undated. https://collections.library.yale.edu/catalog/32458568.

APA, 6th edition

Longstreet, Stephen, 1907-2002. (["Box 4, folder 21"])(undated)[Skin artist]. https://collections.library.yale.edu/catalog/32458568.