MLA

Speck, William A. (William Alfred), 1864-1928. Der Metaphysiker. Box 15, folder 605. n.d.. https://collections.library.yale.edu/catalog/2013135.

APA, 6th edition

Speck, William A. (William Alfred), 1864-1928. (["Box 15, folder 605"])(n.d.)[Der Metaphysiker]. https://collections.library.yale.edu/catalog/2013135.