MLA

Lindbergh, Charles A. (Charles Augustus), 1902-1974. On Excelsior motorcycle. Box 3, folder 16. 1921 June 21. https://collections.library.yale.edu/catalog/10013894.

APA, 6th edition

Lindbergh, Charles A. (Charles Augustus), 1902-1974. (["Box 3, folder 16"])(1921 June 21)[On Excelsior motorcycle]. https://collections.library.yale.edu/catalog/10013894.