The above statement is true, in order for the screw to fit right, the head size has to be longer. A slot is then put in it and then I use Dykem (layout blue)and put this on the head and mark front and back for the orientation of the slot. Back to the lathe and cut to the right depth of the hole and then the slot is put in. I use a jewelers saw that cuts a kerf .026 and has fine teeth.
To my knowledge none of the screws are case hardened. I use Brownell's Oxpho Blue giving it a few coats and then buff them.