MLA

Mason, William Smith, 1866-. Deborah Franklin: letter to Richard and Sarah Bache (manuscript copy); Philadelphia. Box 7, folder 381. 1768 August 16. https://collections.library.yale.edu/catalog/2109674.

APA, 6th edition

Mason, William Smith, 1866-. (["Box 7, folder 381"])(1768 August 16)[Deborah Franklin: letter to Richard and Sarah Bache (manuscript copy); Philadelphia]. https://collections.library.yale.edu/catalog/2109674.