There might be a cardboard like "divider" mounted about 1/2 way down the back of the rear seat, to finish the back where it might be seen above the folded down top if the boot wasn't on! My 66 Valiant is that way!
My GTS never had one and we were likely the first to ever pull the back seat. That's how I found one of the broadcast sheets, lifting the well liner up for a look.