MLA

Mason, William Smith, 1866-. Benjamin Franklin: letter to Benjamin Rush; London. Box 9, folder 525. 1773 July 14. https://collections.library.yale.edu/catalog/2013615.

APA, 6th edition

Mason, William Smith, 1866-. (["Box 9, folder 525"])(1773 July 14)[Benjamin Franklin: letter to Benjamin Rush; London]. https://collections.library.yale.edu/catalog/2013615.