MLA

Fitts, Dudley, 1903-1968. Girondo, Oliverio. Box 1, folder 20. 1943 April 30. https://collections.library.yale.edu/catalog/17122506.

APA, 6th edition

Fitts, Dudley, 1903-1968. (["Box 1, folder 20"])(1943 April 30)[Girondo, Oliverio]. https://collections.library.yale.edu/catalog/17122506.