Hi David,
Interesting pictures! The overwind & U26 winding are subtly different to my V410 one. You have two turns to run the heater
(lucky devil!) whereas I only get one! Certainly is crusty but, like you, I can't see any black areas which are obviously burnt.
I believe the lump in series with the winding is a choke. The two turn winding only needs to generate 1.5V (or whatever the U26 heater is), but the problem comes when the U26 is connected. The heater and those two turns then assumes full EHT potential which will do it's best to arc to the transformer core.
The problem on mine was the connection rivets. I think that's where the leakage started and then that charred the paxolin.
The line stage is obviously working more than well enough to be able to light the U26 heater, so if it isn't then it must be due to leakage to the core.
My suggestion would be to carefully clean of all the crusty bits and then check very carefully for any blackening. Ensure you get continuity through the choke and winding.
I would also suggest wearing disposable gloves to minimise skin contact with the oil- we still don't know that it's not hazardous
Cheers
Nick