Chris, as I also know nothing about 405 TV's (but I have learnt alot form you!) this is my laymans thinking...
The video is showing some smearing, after a burst of white there is a smear of black which should not be there. The bends in the verticals also follow the pattern of the image and if you look carefully, you will see the vertical distortion appears at the beginning of the line. This implies the sync pulse is also being smeared.
You will find an article here
http://graffiti.virgin.net/ljmayes.mal/var/tvsync.htm. It's 625 interlaced but at least shows the sync pulse and back/front porch.
So we are most likely looking at a problem in the RF or video amplifier.