"KOMO, KJR in Seattle Exchange Call Letters", Broadcasting, April 24, 1944, page 72. 950 kHz was a "regional" frequency, with a power limit of 5,000 watts, while 1000 kHz was a "clear channel" frequency, which allowed powers up to 50,000 watts.
"Amateur Radio Stations: Seventh District", Amateur Radio Stations of the United States (edition June 30, 1920), page 77. The "7" in 7AC's call sign indicated that the station was in the 7th Radio Inspection district, while the fact that the "A" was in the range A-W specified that the station was operating under a standard Amateur Station license.
"New Stations: Special Land Stations", Radio Service Bulletin, October 1, 1920, page 5. The "X" in 7XC's call sign indicated that the station held an Experimental license.
"New Stations", Radio Service Bulletin, April 1, 1922, page 4. Limited Commercial license, issued March 9, 1922, to Vincent I. Kraft in Seattle, Washington for operation on 360 and 485 meters for a three months period.
