Def a die break, and a pretty long one at that. I can't tell from your pics where exactly where it goes but from what it looks like to me, I don't see the retained part. I'm not saying it isn't, it does kind of look like it circles back around but I can't tell if it's a crack or just a crease or something in the right spot to make it look that way. An RIDB would be an actual entire piece that is completely surrouneded by a die crack. I don't quite see that here but that's just the best opinion I can give with the info I have. I hope it is though. Still a very nice crack in a place you don't see them that often either.