MLA

Matthias, Blanche (1887-1983). Baldridge, Caroline Singer. Box 4, folder 126. n.d.. https://collections.library.yale.edu/catalog/17170704.

APA, 6th edition

Matthias, Blanche (1887-1983). (["Box 4, folder 126"])(n.d.)[Baldridge, Caroline Singer]. https://collections.library.yale.edu/catalog/17170704.