If i remember right, (was a while ago when i fitted one)
But am sure on the 90 i fitted a seat to, the back rest had 2 locating prongs, which when bolted loosely to the seat back, slide from below in two elongated slot in the tub top rail inner panel,,,, Then a fiddle to tighten all up,
This then held the back rest to the rail, then 2 bolts on the base frame towards the rear, then the base could fold up