Just put the board under the magnifier again, and I saw some black stuff
between Pin7 (the plate) to the common foil.
I scraped at it and now theres a tiny groove you can see in the pix if you
zoom in.
As I rubbed on the nearby ground foil it started to expose the copper
foil...there is a layer of some stuff coating lots of the boards in this
radio , probably niccotine as it was likely owned by an ex smoker. So maybe
its become conductive near this area of the tube and the voltage potential
ignited it, and ate up some of the board.
They did put that pin pretty close to the ground plane in the design.