MLA

Mason, William Smith, 1866-. William Strahan: letter to David Hall; London (includes typed transcript). Box 61 (Oversize), folder 2275. 1769 November 1. https://collections.library.yale.edu/catalog/33490200.

APA, 6th edition

Mason, William Smith, 1866-. (["Box 61 (Oversize), folder 2275"])(1769 November 1)[William Strahan: letter to David Hall; London (includes typed transcript)]. https://collections.library.yale.edu/catalog/33490200.