MLA

Fitzgerald, Robert, 1910-1985. Forbes, Mansfield. Box 12, folder 469. 1931 Nov 20. https://collections.library.yale.edu/catalog/17427379.

APA, 6th edition

Fitzgerald, Robert, 1910-1985. (["Box 12, folder 469"])(1931 Nov 20)[Forbes, Mansfield]. https://collections.library.yale.edu/catalog/17427379.