I have that same wallboard in my upstairs and just this weekend I guttend the entire upstairs. Ours was put in long ago, we found newspapers dated 1940. The board had a sticker on the backside that mentioned it as cork.
The particles in the board were cork and I am guessing they put them in there to make it lighter?
However, I am not certain that the drywall didn't also have asbestos in it. Now I am all freaked out! There is 1" of dust over my entire upstairs. Maybe I will have this stuff tested, but then what good would it do?