MLA

Mason, William Smith, 1866-. Benjamin Franklin: letter to William Strahan, preceded by a copy of his letter to the same dated 1756 July 2; New York. Box 61 (Oversize), folder 2270. 1756 July 27. https://collections.library.yale.edu/catalog/33490115.

APA, 6th edition

Mason, William Smith, 1866-. (["Box 61 (Oversize), folder 2270"])(1756 July 27)[Benjamin Franklin: letter to William Strahan, preceded by a copy of his letter to the same dated 1756 July 2; New York]. https://collections.library.yale.edu/catalog/33490115.