The upright part of a 2door rear seat is different to a Traveller. They are not interchangeable. However, the upright part is the same on 2door and 4door saloons and all saloon rear seat backs DO hinge after a fashion. I have regularly dropped the rear seat in mine to carry longer goods: Full sized gas welding bottles at one time
Rear seats can often be bought quite cheaply secondhand and are usually in better condition as they receive less wear than the front seats. Why not place a wanted advert on here?