I’m not ready to bet my life on it, but I’m 99% certain this is a replacement boot floor. It’s too nice compared to the rest of the car, and the RH outer sill was replaced, and there are no spot-weld dimples on the side flanges, AND look in the corner down by the wheel-well – those other panels are wrinkled but this one is dead-straight. This is probably brazed in from underneath, and while the repair method would not be my first choice, this is a decent job and can stay.