MLA

McClintock, Walter, 1870-1949. Friedrich Wilhelm, Crown Prince of Prussia. Box 1, folder 29. 1908 May 13-1923 October 5. https://collections.library.yale.edu/catalog/2040998.

APA, 6th edition

McClintock, Walter, 1870-1949. (["Box 1, folder 29"])(1908 May 13-1923 October 5)[Friedrich Wilhelm, Crown Prince of Prussia]. https://collections.library.yale.edu/catalog/2040998.