MLA

Purdy, James, 1914-2009. Sitwell, Edith. Box 4, folder 117-20. 1956-59, n.d.. https://collections.library.yale.edu/catalog/32211015.

APA, 6th edition

Purdy, James, 1914-2009. (["Box 4, folder 117-20"])(1956-59, n.d.)[Sitwell, Edith]. https://collections.library.yale.edu/catalog/32211015.