... I'd always taken it as read that when people talked about a thin vertical line towards the LHS of picture as being a result of Barkhausen "effect" or "oscillation" that it meant "Barkhausen-Kurz" oscillation ...
Yes, Colin, that is my understanding, also.
However, rather than being at the left hand side of the picture, in my experience it has always been slightly to the left of centre, which I'd always assumed was about the point where responsibility for the scan changed hands between the output valve and the boost diode ...
When all else fails, read the instructions
I used to explain it away based on the fact that it could be a quarter wave between the diode and the output valve. Sometimes a small choke can be seen on the boost diode cap fitted in order to stop it.
It rings a faint bell with me that a Mazda valve data pamphlet of mine (somewhere!) mentions altered LOP valve geometry to either suppress or shift B-K parasitic.
Looks like I need to sit down and draw out my graphs of V and I vs. line timing around valve LOP stage again- I (sort of) understood it maybe 20 years ago but it's gone very woolly since. I've done very little messing around with tellys in the interim, whilst it was interesting, it was also a bit scary. To me, anyway. Perhaps during the "PL/PY" handover, something odd, abrupt and mischievous happens to electron stream or magnetic action. I'd also noticed the presence of ferrite beads or small chokes on LOP/boost top-caps- OEM's don't spend money fitting things for the hell of it- removing them might have been instructive. Or merely destructive.
As Michael and Refugee highlight, perhaps B-K oscillation is an overly exotic diagnosis to what might be a fairly routine problem. After all, it's a powerful, high slope valve operating over a wide and complex signal excursion, so it would be surprising if there wasn't the odd point where the maths said "oscillate".
Maybe, as with the altered valve geometry case, B-K genuinely occasionally occurs and, having been publicised, becomes a cover-all term for odd vertical picture lines. A little like the way that all vacuum cleaners get referred to as "Hoovers",