[djpateman]

Those are the late 911S calipers from 1972 or so up for the early turbos. They take the 20mm rotor. I believe they would need to be machined for the 24mm rotor. I agree with Ed that you are using the vernier incorrectly. Get the edge of the rotor within the recessed portion of the jaws at least; I also doublecheck my zero often. The S caliper uses thicker pads than the A caliper, but they are otherwise the same. I'm thinking you rotors are not worn much, and may fall within manufacturing tolerance. There are probably places around you that can grind the surfaces; i have done that before. (When I worked in a VW/Honda shop years ago the Honda rotors needed grinding true fresh out of the box; at least Honda said so.)