It should be a typical nut head and it is very much a specialty screw...long, polished past the threaded part and unique. The one in mine is a hex head nut, for what it is worth. I'd guess somebody didn't have that screw, went to the hardware store and got something that sort of fit and as kevinski said added some loctite. Or possibly epoxy. Hard to say. You may have to drill a hole in it and use a screw extractor. Or as you said, grind or file two sides flat and use vise grips. If you have to do that, just place shop rags around and catch any filings. Good luck!