MLA

Mason, William Smith, 1866-. Benjamin Franklin: letter to Thomas Darling; Philadelphia. Box 61 (Oversize), folder 2267. 1746/7 January 27. https://collections.library.yale.edu/catalog/33490089.

APA, 6th edition

Mason, William Smith, 1866-. (["Box 61 (Oversize), folder 2267"])(1746/7 January 27)[Benjamin Franklin: letter to Thomas Darling; Philadelphia]. https://collections.library.yale.edu/catalog/33490089.