MLA

Saarinen, Eero, 1910-1961. Rapson, Ralph. Box 7, folder 75. undated. https://collections.library.yale.edu/catalog/10016202.

APA, 6th edition

Saarinen, Eero, 1910-1961. (["Box 7, folder 75"])(undated)[Rapson, Ralph]. https://collections.library.yale.edu/catalog/10016202.