Why did 405 look so good?

[Murphyv310]

Yes it is a different conundrum Jeffrey. As you have a mains derived prewar set it will easy to see what I'm inferring when adjusting the line speed, disabling the frame timebase will make it more noticeable, you don't need a lot of brightness as it's not advisable with a frame collapse anyway.
I may just have a go with the 1946 Ekco and do a YouTube clip.