MLA

Wilder, Thornton, 1897-1975. Cather, Willa. Box 28, folder 676. 1938. https://collections.library.yale.edu/catalog/16708764.

APA, 6th edition

Wilder, Thornton, 1897-1975. (["Box 28, folder 676"])(1938)[Cather, Willa]. https://collections.library.yale.edu/catalog/16708764.