MLA

Saarinen, Eero, 1910-1961. Pena. Box 12, folder 187. undated. https://collections.library.yale.edu/catalog/10020347.

APA, 6th edition

Saarinen, Eero, 1910-1961. (["Box 12, folder 187"])(undated)[Pena]. https://collections.library.yale.edu/catalog/10020347.