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This came closest. It apparently means that the cushion itself (not the cover) extends beyond the edge of the seat and back rest, a rounded sort of extra bit, making the whole more comfortable. 2 KudoZ points were awarded for this answer
Explanation: My suspicion is that they are square cushions that "unfold", meaning that if if the cushion when folded is 1x1 metre, it unfolds to 2x1 m. I further suspect that the seating in the saloon has some kind of "drawer" apparatus that pulls out to enable it to be used as a spare berth. Therefore, the coussins avec retour would unfold to cover the open berth.