MLA

Mason, William Smith, 1866-. Philip Mazzei: letter to Thomas Jefferson (includes manuscript copy); Paris. Box 15, folder 947. 1780 April 10. https://collections.library.yale.edu/catalog/33353203.

APA, 6th edition

Mason, William Smith, 1866-. (["Box 15, folder 947"])(1780 April 10)[Philip Mazzei: letter to Thomas Jefferson (includes manuscript copy); Paris]. https://collections.library.yale.edu/catalog/33353203.