A quick follow up.
Voltage output gets very stable once the voltage has been selected and after a while the long term drift seems to be minimal as seen on my HP3478A. After setting it up as a test to 13.80V the output was 13.8101V, and after about 40 minutes it was 13.8040. I then turned off the PS to go out (but left the DMM switched on), and turned it back on 5 hours later, and the output voltage was 13.8047, , and 90 minutes later was 13.8042, another 90 minutes then 13.8041, and then after 11 hours of staying on overnight in the morning it was 13.80040. So long term stability seems quite nice.
I am now running the same test as above while the PS is set to output 2.50V, and this will also give me an additional 5th decimal resolution. I haven't pulled out yet the HP3457A that is in storage, and that would give me 6 decimals. Maybe during next week.
My main issue so far is the zero output adjustment. I can set it fairly easily (it a 10T pot adjustment) within 15-20uV or even better (spec says within 50uV) of zero output, but if I then set any output voltage with the controls, and then go back to zero, the "zero" output voltage will have raised to 1.5 to 2mV or thereabouts and will not change much from there even if given time to settle. And if at that point I try to readjust the offset back to zero I will even run out of range on the potentiometer. But if Instead I just turn off the power supply for about a minute and then turn it back on, the offset voltage will once again have fallen to uV levels that I can once again set properly. If I set the output to zero, and adjust it properly, then just turn off the power supply, and then turn it back on, it will correctly come back to the level I had set the zero adjustment.
So I wonder if the OP-05CP Opamp used in this design might have gotten unstable over all these years (and may just sat unused for a good number of those) and is no longer able to maintain stable offset. Datasheet of the OP-05 says its obsolete, recommended substitute is OP-07. But according to its datasheet, the 07 has actually worst drift specifications: drift/temp: 1.3uV/deg-C max, drift/time: 1.5uV/month, against the 05 specs which has drift/temp: 0.5uV/deg-C, and 0.2uV/month.
Seems like it would be better to try with a OP-05CP old stock device first, and if not then the current 07CP. I realize these are all minimal differences and I may never take real advantage of these, but just in the spirit of restoring equipment to its rightful original specs I would like to try and fix this drift issue. On the other hand, perhaps its not the IC that is causing the drift, but something else in the circuit. Still have to take a close look at the board, but I have to say that everything looks absolutely perfect.
Opinions and suggestions welcomed. I have attached the schematic of the power supply, and both datasheets of the 05 and 07 op-amps.