When I started work, sometime in the 1980s, we had the DC only version of this meter. It was the only half-decent bit of kit that we had! The "Cal" position measures the voltage on the standard-cell and there's the screwdriver adjustment to bring it into alignment. I had always assumed that the standard was some sort of compensated zener (or such like) - never dreamed that they would put a standard cell in the unit!