Things you see when you are crawling around under your car!

I don't think the factory had a-body rear sway bars till like '76, and they were on fleet vehicles, like cop cars, taxis, etc. IIRC, those brackets attached to the sides of the frame rails. But, who knows, it could have been an aftermarket sway bar, or maybe something PO adapted to the car. I know the a-bodies can use that rear sway bar, because I've used them.