This stuff was tougher than tough......... may have been some damage to the house during WWII as a V2 had a direct hit on a block in the flats opposite killing 44 people, from what I understand one of the neighbours lost the front bay of their house when the aftershock came though, thus after the war it could have been installed.
It could well be a cement board, which may contain asbestos. It was commonly used through the 30's - 70's. This is why I mentioned it and why I have mentioned it previously about other things in other threads. There is nothing wrong with leaving asbestos containing materials in place so long as they are undamaged, managed (kept in good condition or encapsulated) and/or left undisturbed. Its when they get damaged, cut, ground or drilled which releases the dangerous fibres into the atmosphere. What people may not realise is that the majority of the dangerous fibres are so small you cannot see them with your naked eye.
