MLA

Wilder, Thornton, 1897-1975. Curtius, Ernst Robert. Box 31, folder 749. 1949, n.d.. https://collections.library.yale.edu/catalog/17056643.

APA, 6th edition

Wilder, Thornton, 1897-1975. (["Box 31, folder 749"])(1949, n.d.)[Curtius, Ernst Robert]. https://collections.library.yale.edu/catalog/17056643.