MLA

Thayer, Scofield, 1889-1982. Hugo von Hofmannsthal. Box 69, folder 1867. n.d.. https://collections.library.yale.edu/catalog/17251477.

APA, 6th edition

Thayer, Scofield, 1889-1982. (["Box 69, folder 1867"])(n.d.)[Hugo von Hofmannsthal]. https://collections.library.yale.edu/catalog/17251477.