Well Ed, I would say it is a Philips screw, and Andrew Laszlo obviously liked to use them as there is one on this other prototype.
Unfortunately the pictures Robert Beeman sent me were fairly low resolution, so I couldn't make out if it was writing or just scratches on the screw head when I blew the picture up.
The trigger guard on the commercial Hy-Score is machine pressed and has a curved cross section. As far as the prototypes go, the trigger guards look like a quick bending of some strip steel in a vice.