For its first 14 months of operation the Detroit News' broadcasting station had an amateur radio licence, callsign 8MK, transmitting on a wavelength of 200 meters (1500 kilohertz). The switch to operations using a Limited Commercial licence resulted in a new callsign, WBL, and a change to 360 meters (833 kilohertz).
Detroit News, October 30, 1921, page 10:
Detroit News Radio Department column
    The Detroit News radio station has been issued a limited commercial license, call letters WBL, which will used as soon as a new antenna can be erected to handle the increased wave length. The working wave will be about 360 meters. The station will continue to operate on its present length until after all tests have been made, at which time an announcement will be made over the radiophone advising the time that the permanent change will be made.

    F. E. Babington: You will find elsewhere in the department an improved regenerative circuit which has been used at 8MK for the past few months and found to be satisfactory in every respect.
November 6, 1921, page 17:

    The Detroit News radio station is now operating under a limited commercial license call letters, WBL. The wavelength used is 360 meters and a special antenna has been erected to use this wave. The station will transmit as before beginning every evening, except Sunday, at 7 and finishing at 8. Reports regarding this change will be welcomed.
November 20, 1921, page 28:

    H. C. R.--A limited commercial station is one that is operated by some business concern and is used to carry on company business with. There are a number of limited commercial stations in operation for broadcasting news, market reports and concerts. These stations are not permitted to handle general public correspondence.