SR = Sakae Ringyo, an innumerable quantity of 80's bikes were built with SR cranks, stems and seatposts. The products range from awful steel-ringed swaged stuff to really nice, light, forged cranksets.

Have you tried just turning the cup with your fingers? If the lockring is already off it should move pretty easily, unless you're about to have a bad day. Try putting a screwdriver in the notch like you already mentioned but just tap it with the palm of your hand instead of a hammer. It may just have a little dried grease in ye olde threads.