On my 2003 38 Open the curved seat top is 2 pieces. One, the side piece, covers the top port side and the 2nd covers the piece facing aft... Both pieces are removed by opening the zipper on the bottom of the top pieces allowing access to bolts that hold the top pieces to the brackets supporting it....Your seat on the 31 may be different but assume it's somewhat the same Best I have.. M2C
If I remember correct you will have to go under the seat inside the hatches and unbolt the bottom cushion first. Then you should have access to the brackets holding the back down.