MLA

McNally, Rand, and Company. Map of the Montana State Highway System / Prepared by Rand McNally & Company. State Highway Commission, 1942. https://collections.library.yale.edu/catalog/15818318.

APA, 6th edition

McNally, R., & C. (1942). Map of the Montana state highway system / prepared by Rand McNally & Company. [Road maps]. State Highway Commission, https://collections.library.yale.edu/catalog/15818318.