MLA

Koch, Frederick R. (Frederick Robinson). Puccini, Antonio (to Angelo Magrini). Box 50, folder 1051. 1924-25, n.d.. https://collections.library.yale.edu/catalog/16101624.

APA, 6th edition

Koch, Frederick R. (Frederick Robinson). (["Box 50, folder 1051"])(1924-25, n.d.)[Puccini, Antonio (to Angelo Magrini)]. https://collections.library.yale.edu/catalog/16101624.