"New Stations", Radio Service Bulletin, November 1, 1924, page 3.
"Fourth District", Amateur Radio Stations of the United States, June 30, 1924, edition, page 96. The "4" in 4RU's call sign specified that the station was located in the fourth Radio Inspection district. The fact that "R" falls in the range A-W indicated that the station was operating under a standard amateur station license.