1965 Valiant bench seat seatback issues
The 'hook' in the center looks bent down a bit, also, I've seen people drill a hole and use a pin on the outer hinge because the metal caps pop off, but it looks like the hole/pin are too far out on the stud causing the seat to slide away from the center pin.
Also, there should be a hard plastic stop screwed to the bottom of the seat back, and a corresponding metal stop screwed to back of the seat bottom, that stops the seat from going backwards.