Beaver Board or Building Board was all I remember it being called, 60's or 70's and pretty sure has not been made in many years.

We used to call it "stucco board" not to be confused with today's stucco board, made from cement. It was used for exterior renos to mimick stucco, but without its durability.

I think it is a plaster It can be applied over concrete or cinder block, brick, stone or wood. It creates a durable, hard surface that conforms to flat or curved walls and ceilings. Plaster can last for hundreds of years, but has one major enemy that will cause plaster walls to become powdery.

Here is a link that might be useful: Modern House Plans

It looks like beaverboard or blackboard--a fiberboard of wood bits and other scraps. Non hazardous I believe.

It is impossible to be sure if this is a poorly textured wall finish or paint on a wood fiber board as others have suggested.

You need to do more work to find out if the material is worth saving. I would cover it with 3/8" GWB.