MLA

Mason, William Smith, 1866-. Benjamin Franklin: letter to all captains of armed vessels, safe conduct for William Gray (includes typed transcript and manuscript copy); Passy. Box 13, folder 803. 1778 July 6. https://collections.library.yale.edu/catalog/33264071.

APA, 6th edition

Mason, William Smith, 1866-. (["Box 13, folder 803"])(1778 July 6)[Benjamin Franklin: letter to all captains of armed vessels, safe conduct for William Gray (includes typed transcript and manuscript copy); Passy]. https://collections.library.yale.edu/catalog/33264071.