Starrett used to make screw slitting hack saw blades. They had no set,like Tony describes. I wish I could find some. I saw a set of them donated to the museum years ago,but the gunsmith shop nabbed them. I will hacksaw a groove,and clean it up with needle files or very thin files. For very fine screw slots,like found on high class European antique guns,I saw them with a large jeweler's saw blade(about .020" wide),and they make very neat screws.