MLA

Fitzgerald, F. Scott (Francis Scott), 1896-1940. [Letter : 1939 September, F. Scott Fitzgerald to Gerald and Sara Murphy]. Box 3 | Folder 55. 1939 September. https://collections.library.yale.edu/catalog/11859303.

APA, 6th edition

Fitzgerald, F. Scott (Francis Scott), 1896-1940. (["Box 3 | Folder 55"])(1939 September)[[Letter : 1939 September, F. Scott Fitzgerald to Gerald and Sara Murphy]]. https://collections.library.yale.edu/catalog/11859303.