MLA

Williams, William Carlos, 1883-1963. Antheil, George. Box 1, folder 23. 1931-1932, undated. https://collections.library.yale.edu/catalog/17049444.

APA, 6th edition

Williams, William Carlos, 1883-1963. (["Box 1, folder 23"])(1931-1932, undated)[Antheil, George]. https://collections.library.yale.edu/catalog/17049444.