Lots of strange building materials in that era.
Seen wood fibre boards ceilings with lining paper on also from that era!
Essex board also known as tintest board, it was manufactured local at Marks Tey, it was used as a cheap repair to ceilings in war time.
it was still in production in the 60s it was used on ceilings upstairs in council housing, the more expensive plasterboard down stairs.