Bucket Seat Back Mounting

Okay, I am at a complete loss now. I assumed that the lower portion of the bucket seat backs on my 69 Dart were screwed into the seat frame. Upon further investigation I realized that the holes on the seat frame are not actually threaded. So now I am at a loss as to how exactly the backs mount (and stay on). I understand how the four L-shaped brackets slide under the frame mounts on the back of the seat, but what keeps the seat back bottom from popping off and causing the whole back to fall off? Any pictures that you have showing how exactly they are mounted would be greatly appreciate.