There's a spring loaded pin at each bottom corner. The outer end of the pin has a sort of hammerhead shape. The hammer head fits into a slot built into the wheel well and drops down to the bottom of the slot. The top edge of the slot is bent in to act as a stop. The spring loading is to the inside--not outward---so the trick is to get a bent ended tool with some leverage (it is a strong spring) to push the hammerhead outward so while lifting the seat out the hammerhead clears the bent-in sheetmetal. Lifting the seat (heavier than you'd think)out and manipulating the pins (one side at a time)is best done with an assistant. If you're alone have something handy to keep the now released seatback up--otherwise it will drop back into the slot---I know this for a fact.
I have a dark brown seatback from an '86 (they can be updated and backdated--they all fit -- currently my '80 245 has a seatback from a '76 265) available and can help you install it--I'm in Bay Shore. Give me a call--six-three-one-968-8933 -- Dave