MLA

Wilder, Thornton, 1897-1975. Hemingway, Ernest. Box 41, folder 1051. 1929-31, n.d.. https://collections.library.yale.edu/catalog/17056649.

APA, 6th edition

Wilder, Thornton, 1897-1975. (["Box 41, folder 1051"])(1929-31, n.d.)[Hemingway, Ernest]. https://collections.library.yale.edu/catalog/17056649.