I've come to the conclusion that this foam was deliberately added by the
manufacturer, perhaps instead of a buried thermal fuse. The minimum setting
of my temp controlled soldering iron is too high for this but blipping on
and off the mains showed that smoke comes off this foam starting at 150 deg
C and copiously by 160 deg C.
So it is going back under the Tx clamp , snipping off a bit for my
reference.
It is closed cell from viewing under x30 and certainly does not act as a
sponge with water, with gas/smoke unknown. At this stage I'm pretty
confident that the foam was preloaded with tar or something like that ,
rather than a malfunction of the Tx despite high temp running. The coachbolt
nut was probably loose from creep of the windings at this elevated temp as
much as foam compression