The cover is tucked into the sides, just like the W123 and W115. I
think you have to pull the bottom of the back panel outward some, then
down. It may have stuck to the covers -- a thin bit of blunt plastic
slipped along where they meet will help separate them. Going back in,
tuck the cover in, the slide the panel back in.
I've not had mine apart (at it won't be hard, someone else did it for
me a screwed them up), but a friend of mine said his were pretty stuck
at first.
The plastic panel doesn't flex as easily as the other fiberboard ones
did.
Peter