MLA

Gascoyne, David, 1916-2001. [Smart,] Elizabeth. Box 11, folder 255. undated. https://collections.library.yale.edu/catalog/2055520.

APA, 6th edition

Gascoyne, David, 1916-2001. (["Box 11, folder 255"])(undated)[[Smart,] Elizabeth]. https://collections.library.yale.edu/catalog/2055520.