MLA

Gascoyne, David, 1916-2001. Caetani, Marguerite. Box 10, folder 169. undated. https://collections.library.yale.edu/catalog/16591647.

APA, 6th edition

Gascoyne, David, 1916-2001. (["Box 10, folder 169"])(undated)[Caetani, Marguerite]. https://collections.library.yale.edu/catalog/16591647.