7jp4 crts

[Electronic M]

The 621 was perhaps the only 7" set that used a magnetically deflected CRT (7dp4).

Nearly all other 7" sets of the era used the electrostatically deflected 7jp4 or 7bp4, and adapting a electrostatic set to use an electromagnetically deflected CRT or vice versa is no trivial matter(it would require a total redesign and rebuild of the deflection and HV circuits of the set in question as well as the sourcing of scarce specialized parts).