MLA

Speck, William A. (William Alfred), 1864-1928. ALS : Ludwigsburg, to Christian Gottfried Körner. Box 15, folder 603. 1793 Sep 15. https://collections.library.yale.edu/catalog/2013133.

APA, 6th edition

Speck, William A. (William Alfred), 1864-1928. (["Box 15, folder 603"])(1793 Sep 15)[ALS : Ludwigsburg, to Christian Gottfried Körner]. https://collections.library.yale.edu/catalog/2013133.