Got this from a friendly 245 owner in WA. Seat design is unlike one I have seen. There is no hook plate. What are the brackets with the black plastic hook things, do they somehow function as hook plate? Am I meant to build a brace for the two thinner cushions? I set them up just how I think it would fit together... Any help would be appreciated.

Other photos included are of different brackets. I'm a little lost. Any one else ever see this design?