MLA

Speck, William A. (William Alfred), 1864-1928. Wallensteins Tod. Box 15, folder 607. n.d.. https://collections.library.yale.edu/catalog/2013137.

APA, 6th edition

Speck, William A. (William Alfred), 1864-1928. (["Box 15, folder 607"])(n.d.)[Wallensteins Tod]. https://collections.library.yale.edu/catalog/2013137.