"8zz" was the initial callsign used by of Westinghouse for its new broadcasting station, with began regular service with election returns of the night of November 2, 1920. That broadcast service soon switched to the callsign of KDKA. The callsign of the experimental station operated by the Precision Equipment Company was 8XB.
Cincinnati Post, October 29, 1920, page 11:

    Westinghouse Electric and Manufacturing Co. will send a test wireless telephone call between 7 and 8 p. m. Friday from its station at E. Pittsburgh, Pa., in preparation for flashing of election returns Tuesday night. Precision Equipment Co. will relay the results from Cincinnati. The call will be 8zz.