What the heck is a "radiator plate" in a CRT based oscilloscope?
I think they're referring to the deflection plate connections &mdash; pins 8 and 10 on the CRT, which connect to R474/L407 and R475/L408, respectively, on the vertical output amp board X73-1220-00. This is supported by the fact that if they are "NG" (no good), then the chart concludes that the vertical amplifier is defective.

Similarly, the next step has you checking pins 10 and 11 on a different board (it says X74-1210-00, but I think they really mean X74-1080-00). These would be the horizontal deflection plates (pins 12 and 13 on the CRT), because if they are NG, they have you focusing on the horizontal amplifier.