I encountered some sheets of the stuff when I did some renovation of
my late 19c home...obvious to me from a later renovation where
plaster was replaced.

I don't know what's in it, but it appeared to be about as dense as
plasterboard but not as white, and not as thick, but thicker than
masonite.  If I remember correctly, it had a paper back printed with
a beaver.

--The Rev.