I think that's the bumper from the TWR body kit that was briefly offered here in the US towards the end of the Classic body style. It was a specially edition available in the UK but I don't really know if any Range Rovers were actually imported with the kits installed here in the US or if is was a dealer installed option. The kit included sills and a wrap around rear bumper as well. In the UK I think there also might have been an upper air dam. I've only seen photos of that particular piece in James Taylor's various books.