MLA

H. D. (Hilda Doolittle), 1886-1961. Heydt, Erich. Box 54, folder 1373. 1956, n.d.. https://collections.library.yale.edu/catalog/2066735.

APA, 6th edition

H. D. (Hilda Doolittle), 1886-1961. (["Box 54, folder 1373"])(1956, n.d.)[Heydt, Erich]. https://collections.library.yale.edu/catalog/2066735.