MLA

McAlmon, Robert, 1896-1956. Hemingway, Ernest. Box 1, folder 37. 1923-25, undated. https://collections.library.yale.edu/catalog/10268432.

APA, 6th edition

McAlmon, Robert, 1896-1956. (["Box 1, folder 37"])(1923-25, undated)[Hemingway, Ernest]. https://collections.library.yale.edu/catalog/10268432.