[hamjet]

In the 2nd photo it looks like a locking tab with a piece of it bent up to stop the nut from loosening. try cleaning it better with a wire brush to see if I'm correct. you will have to flatten out the tabs around the bottom if thats what it is.. I don't see signs of heat on the nut you say you heated, you need more heat( a hotter torch) if it won't break loose after bending down the tabs. sometimes a breaker bar with a hard wack of a BFH on the bar will shock the nut enough to break it loose....