Warding or keyhole files are old as the hills. Just not common any longer as hand working has long gone away for the most part. Mark's description is a good one, I'll add that the ones I've had seem to have a double or bastardish cut to them. They also have some lengthwise flexibility that more common files lack, for better or not. My understanding was they were for creating slots out of drilled holes. I have a range of sizes and like them a lot for slotting stays and blades for tabbed dropouts too. Andy