My fault. I was reading the snippets that lead in to the website I captured that and pasted it. The actual website doesn't make that claim.
www.myvintagetv.com/picture_tubes.htm
Probably was used to experiment with magnetic deflection. .... 7DP4. This CRT was primarily used in the RCA 621 set. Magnetic deflection; magnetic focus. ... This CRT is electrically identical to the 7JP4 except for the screen size. Electrostatic ...