I have both a DAS 46A and a TSC 46 and both had minor issues when I bought them-several months apart and from different EBAY sellers. Before messing with the contacts in the Kelvin-Varley divider, you should take a look at the power supply board and the Chopper-Stabilized Amplifier board. Towards the left side of the power supply board is a daughter board which is actually the Chopper-Stabilized Amplifier. It's very easy to miss the fact that it is socketed onto the power supply board. You can see the CSA daughter board in your tear-down picture
https://www.eevblog.com/forum/testgear/general-resistance-das56a-tear-down-and-repair/?action=dlattach;attach=265549 towards the left side of the power supply board.
My DAS 46A had very unstable output readings until I lifted the corners of the CSA board and re-seated it onto the power supply board. It was not necessary to fully lift it. After re-seating it, the output was very stable and very easy to zero with the 10-turn pot at the top edge of the power supply board. Short circuit the meter's leads and make sure it shows '0.' Then set the Kelvin Varley to zero, connect the meter and it should also show zero-well, adjustable to within 5 micro-volts of zero.
As an aside, I'm using an HP 3455A bought through EBAY and repaired through several weeks of finding a few minutes at a time to learn how some portions of the circuitry worked. I've bought four of them (different auctions) in non-working condition and currently have two of them working.
The second thing I did to get my DAS 46A reliable to use was to take advantage of what I learned with the TSC 46. Although it has additional circuitry for thermocouple calibration, in one mode it fully duplicates the DAS 46A as a voltage reference. There is very little inside the transformer enclosure-just some bridge rectifiers and 1 Micro-Farad capacitors-so I wouldn't go in there just yet.
Instead, look at the power supply board. It uses a clever circuit which only needs only one 1N752A zener diode as a reference for both the + and - 15 volt power supplies that feed the CSA board. I believe that tries to ensure that both + and - 15 volt outputs drift together, improving stability of the CSA.
Another voltage regulator circuit provides 1N752 zener stabilized power to the two series-connected 1N827 zeners in the other 'box' located below the Kelvin Varley divider. That also looks like some gray-beards implemented a very carefully orchestrated circuit with a lot of selected & matched 0.01 % resistors used to partially offset the 1N827 temperature drifts. The care they took is incredible. On my TSC 46, I first replaced only the one 50 Micro-Farad capacitor (with a 220 Micro-Farad) which had failed some time before I had bought it. Replacing the one capacitor brought it back to life. I used it for many weeks while working on one of the broken 3455A meters, but then one day the TSC 46 became unstable again. The other 50 Micro-Farad power supply capacitor (-15 volt side) had failed. So, I replaced all of the 50 Micro-Farad capacitors with 220 Micro-Farad capacitors just to avoid having to do it piece-meal. They were vintage 1979, so that was not a bad service life.
I don't have anything good enough to calibrate my 3455A meters. But I did buy one of those LM584 references on EBAY which had been measured by the seller's 'still in calibration period' Agilent 34401 to give me a ball-park check. I set the DAS 46 to match the seller's 34401 measurement at each of the four settings and made the following table.
Note the LM584 reference outputs were supposedly 2.49980, 4.99829, 7.50249 and 10.00001. The third column shows the readings of the LM584 reference from my repaired 3455A.
DAS 46 Setting 3455A "LM584" Reference per 3455A:
2.49980 2.49976 2.49977
4.99829 4.99824 4.99832
7.50249 7.50241 7.50252
10.00000 9.99989 10.00010
While I don't know truly how close the DAS 46 is to 'spot-on,' I do have some confidence that it is not terribly far off. To those who designed and built the General Resistance devices-you should be very, very proud of yourselves.
So, I'd start by reseating the CSA daughter board and replacing the 50 Micro-Farad power supply board capacitors before I'd take apart the gold-plated Kelvin Varley six-decade divider. Put an oscilloscope on the voltage regulator outputs and make sure they are not noisy. The CSA can't give clean output without clean power.