From 1946 to 1951 the 7JP4 was a common CRT (Picture Tube or Kinescope) used in lower priced televisions sold in the United States.
For a detailed look at US electrostatic TV design and the 7JP4 Kinescope from an Australian view (by cablehack) go to the external link below involving the Wards Airline TV (Sentinel model 400TV) [REF].