There is a "bumpy" ring on the back side of the rotor that is held on by three allen bolts. As the ring spins with the rotor it passes by a sensor that is stationary and counts the little bumps as they go by and tells the sensor if the wheel is spinning faster or slower than the others, or if it's locked up during a skid. The picture below is from when I replaced my rotors a couple of years ago. You can see the darker colored tone ring on the new rotor.



I'm guessing that you could get one from Merlin at St. Charles Isuzu, or maybe you could talk to one of the guys parting their VX's out to see if they still have the original rotors and could get the ring for you. Either way, I would go back to mechanic boy and either tear him a new one, or make him do the work for little to no cost.