There should be an insert cushion. In some boats, that berth is rarely used, so the cushion ends up in storage. Then when the boat is sold, it is forgotten about. One of the previous owners more than likely has/had it.
I wish they had designed the backs of the settee to be removed and used as the filler. It would make the berth longer and allow you to carry and store the filler. With a separate insert it always seems to be in the way.