I had read that 1500-1600 kHz weren’t added to the band until the North American Broadcasting Agreement of 1941 (see http://en.wikipedia.org/wiki/North_American_Radio_Broadcasting_Agreement ).
I’d like to hear more about what was actually meant by “high fidelity” in regards to KHFS’s operation. Other stations in that era and previously (such as WHAM) had loosely used the term in describing their operations, but without some numbers, the term becomes somewhat meaningless. Nonetheless, I had been under the impression that AM broadcasters of the 1950s era did not use any deliberate means to control frequency response and as a result the bandwidth of the transmitted audio was just a function of the bandwidth of the transmitter, the studio-to-transmitter link, and any transformers that might have been in the audio path.
I once saw a frequency response plot for a late 1930s era 50kW AM broadcast transmitter, which was rated for a response from 50 Hz to 10 kHz (+/-1 dB, if memory serves me correctly). The same book presented the response curves for several 1920s era transmitters, all of which had roll-off below 200 Hz or so and above 4 kHz or so). Needless to say, I was surprised to see that 1930s era broadcast equipment could perform that well.