sofa

I want to re-cover the sofa in my 2720SL with new fabric. My 2001 has the bench seat design, but I notice that the 2005's appear to have separate left and right cushions in both the seat and the back, a design which I prefer.
Can someone with a later model please explain how their sofa is constructed? It seems to me that since the sofa turns into a bed, there must be a single piece of wood behind two cushions. So are the cushions actually separate from the wood, or are there, say, two pieces of foam attached to the wood and the fabric goes round the foam and the wood and into the groove between the two foam pieces, or something similar?

Any help would be appreciated.
Paul