Troppo the Hindenburg Wall is in the Western Highlands. I have flown a LOT around the area of that map and I am sure I have seen the feature asked about - but then spectacular waterfalls and river rapids etc were VERY common in the PNG highlands so I have no specific memory of those ones. I do remember a HUGE river/rapids west of there that just disappeared down a huge hole and went underground for miles before reappearing. Such is the nature of limestone formations which abound in PNG.