I've repaired, restored, tweaked and tuned a few of these and while I have used them for calibration purposes beyond what you might expect, it was always with a more precise meter in parallel and it was challenging, like using a micrometer aboard a boat in rough weather. There are a few improvable issues regarding stability, such as replacing a commonly faulty relay in one of the control loops and also carefully adjusting those loops. Calibration and adjustment of these are much more involved than just verifying the output accuracy. For best stability you need to let them warm up for hours on end--like overnight--and then let them stabilize after each output is selected, often for several minutes.
And after you do all that, they are still nowhere near what you need for actual standalone professional calibration of 6.5 digit meters or even the better 5.5 digit models. These are 50ppm class devices and that's for DC on a good day. The AC part is 350ppm, which is actually pretty good, but stability on the higher voltages is problematic.