MLA

Speck, William A. (William Alfred), 1864-1928. ALS, to Friedrich Justin Bertuch. Box 9, folder 269. [1789?]. https://collections.library.yale.edu/catalog/2005299.

APA, 6th edition

Speck, William A. (William Alfred), 1864-1928. (["Box 9, folder 269"])([1789?])[ALS, to Friedrich Justin Bertuch]. https://collections.library.yale.edu/catalog/2005299.