Hi again.
To finish off this thread.
For whatever reason, the 'spare' CRT damaged the video board, revealing that the CRT is dead and may well have been why the 2nd 492 had been stripped down so much.
In the end I didn't open up the YIG, as I was unable to locate any clear images so I focused on the LV rails (which is where I should have started in the first place). The +9v rail was reading +1.26v and the +9v rail 2A fuse was open.
One thing that had also changed following the failed CRT test was that the scale illumination no longer worked.
Swapping around the multi-pin illumination connector situated behind the CRT bezel resolved that problem, resolved the blown fuse and also resolved the lack of vertical deflection.
With the unit fixed and working again, I packed the SA away and have now passed it on.
The End.
James
James55