Stanton,
I too have noted many with tapered slots (in German guns). In observing the screws/pins being fitted in the first place I noticed the following. Most had a sacrificial slot that was filed away in "clocking" them. If the final slot was filed, it was usually tapered, but if sawed with a jewelers saw had straight sided slots. I also noted that most screwdrivers/turnscrews were self-made, mostly from worn out Pilar files and also most were twisted like a corkscrew, to guard against breaking.
Mike