Guys, thank you for the feedback.
I think D411 next to metal 750K resistor is actually part of current measurement, not resistance. I will try to verify it. The actual board is much cleaner now than what you see in the picture. I even have a replacement part for it. However it was so close to precision network, I was not sure if I can remove it without accidentaly touching it with soldering iron. I may have to try if everything else fails.
I just ran few more tests. My resistance standards are not the greatest, some are +/-100ppm, some are +/-20ppm.
2k range off by 30ppm
20k range off by 150ppm
200k range off by .25%
2M range off by 2.5% this is after 1 hour warm up. It is much worse before warm up.
20M range off by 30ppm
200M range off by 30ppm
1G range off by 40ppm
Seeing that performance improves at 20M, I doubt it is leakage that is responsible for the problem.
I also checked bias voltage with 10M resistor and got -0.3mV
I see the pattern with two parallel diodes pointing in opposite direction is used in at least 3 places:
CR210, CR211 (by that DG411)
CR212, CR213 (by large rad COTO relay)
CR214 - actually 2 diodes in one case (by large rad COTO relay)
I think it is used for some sort of voltage clamping. Would someone know what exact use is? Thanks.