[Arran]

OK sorry if my question seemed confusing, I was referring to the black insulation pads glued on the the inside of the cardboard back cover. I assume its for sound since its not near the chassis so it doesn't deal with any heat.

If it's for sound it may be some form of rock wool, it was common to use that for sound deadening insulation in those days, it still is for that matter. Rock wool is made from rock slag leftover from steel plants, there isn't any asbestos in it as far as I know?
Regards
Arran