MLA

Fitzgerald, F. Scott (Francis Scott), 1896-1940. [Letter], Tryon, North Carolina [to] Gerald and Sara [sic] [Murphy]. Box 3 | Folder 55. 1937 January 31. https://collections.library.yale.edu/catalog/2019895.

APA, 6th edition

Fitzgerald, F. Scott (Francis Scott), 1896-1940. (["Box 3 | Folder 55"])(1937 January 31)[[Letter], Tryon, North Carolina [to] Gerald and Sara [sic] [Murphy]]. https://collections.library.yale.edu/catalog/2019895.