It looks as if someone screwed OSB onto the studs and then maybe skim coated it with plaster? I've never heard of this before and am not sure how to proceed. This is probably why I have hairline cracks in some areas of my wall. I expected those in my old house where all the walls were lathe and plaster, but here I thought the walls were all drywall! Guess not!
And the plaster was crumbling off the wall as I removed the baseboards, even though I was trying to be very careful. It didn't help that the baseboards were glued onto the walls as well as nailed, so pulling them off caused chunks of plaster to come away.
This cut and dry job just got a little more complicated! On the bright side, I did find this lovely wallpaper hiding behind the baseboard. It appears to have some plastic or vinyl in it, classy!