MLA

Pearson, Norman Holmes, 1909-1975. Pratt, Margaret. Box 93. [1956?]-1974, undated. https://collections.library.yale.edu/catalog/17260079.

APA, 6th edition

Pearson, Norman Holmes, 1909-1975. (["Box 93"])([1956?]-1974, undated)[Pratt, Margaret]. https://collections.library.yale.edu/catalog/17260079.