I found a “wideband/doublewide” channel reference:
“On December 19, 1933, the Federal Radio Commission authorized three new broadcast-band channels above the previous upper bound of 1500 kHz. The three channels, at 1530, 1550, and 1570 kHz, were 20 KHz wide instead of the standard 10 kHz, thereby making high-fidelity broadcasting possible without interference from adjacent stations.”