I have run into these while taking engines from MG sports cars. We had it down to a science when we were stripping the block. We found some tiny left handed drill bits, and the what we were calling rivets came right out. If we didn't remove these the aluminum engine number plates were usually melted during the cleaning process. Of course we ran into those that the heads were sheared off for what ever reason. Usually some heat and lefty bits would get things out.