Andy,
Thanks for the information in the photographs. I would agree with David and Andy MOYFA that it looks like ruberised organic fibre but it would be worth having it checked; I think your local authority should have an asbestos officer who should be able to advise you and this may be some use
http://www.hse.gov.uk/asbestos/gallery.htm.
At the risk of sounding facetious, which I don't intend to be, have you asked AVO? i.e. Is there anything, such as a parts list, in your records? I would expect that for a ministry contract there would once have been very full documentation.
Andy MOYFA,
I can empathise with your views on equipment quality having "grown up" in the world of broadcast equipment which was considered to be second only to military standard, and then sometimes grudgingly. Medical equipment would be in a similar class.
Sadly those few manufacturers who attempted to set high standards usually seem to have found it uneconomic. I am thinking of a Bush television set which was constructed as a circular cage with four pressed aluminium sub-chassis which I remember as having corresponding cover plates. I was told that they had drawn on experience of making military radar sets but I have no way of knowing if this was just speculation.
PMM