Hal,
The screw should be above the surface and in my experience is not threaded to the end, so it can't be screwed below the surface.It seems that yours is missing(if you are lucky) or twisted off below the surface(if you aren't).A proper screw above the surface can be turned by a thin blade screw driver.Your problem reaching the screw seems to be becaues it is missing or broken.If it is missing, a new one can be turned in. If it is broken off, the trigger plate will have to be removed and disassembled to remove the broken piece.I believe New England Custom Guns can help you with this, or maybe Turnbull.
Mike
