MLA

Speck, William A. (William Alfred), 1864-1928. ALS : Zurich, to Christoph Girtanner. Box 12, folder 461. 1785 Mar 8. https://collections.library.yale.edu/catalog/17266943.

APA, 6th edition

Speck, William A. (William Alfred), 1864-1928. (["Box 12, folder 461"])(1785 Mar 8)[ALS : Zurich, to Christoph Girtanner]. https://collections.library.yale.edu/catalog/17266943.