It's almost scary how stable and accurate they can be. The later PD precision supplies have the LM399A reference, which is the same zener reference used in the HP 3455A, 3456A, and 3457A multimeters, so as long as the precision resistors in the supply don't drift, they should remain every bit as stable as those 6.5 digit meters.
I haven’t figured out the whole story, but the original PD company was more than likely bought out. It isn’t hard to notice that the quality of Power Designs small precision bench supplies went downhill dramatically with models like the 2040A and 1001A. These were encased in cheap ABS plastic cabinets and had meters, knobs, and binding posts that were, well, horrible to put it lightly. They also used a, “dicky,” as Mr. Jones would put it, plastic thumb-button assembly to select the output voltage. This led me to think there was no way the original PD company would put anything like that on the market, because it was in such stark contrast to anything they made in the past. Looking at the manuals for these later PD supplies, the company is no longer in NY or CA, but listed at 14 Commerce St, Danbury CT- most likely the address of the new owner, thankfully no longer in business making this sort of garbage. They obviously lacked the superb metal working capabilities or any concern to retain the type of quality of the original PD power supplies.
-Mark-