Perhaps the reason the tenths of thousands is "off" a bit is because of the connections. Banana jacks can be problematic and there may be voltages induced with different metal to metal connections. Perhaps try clean good solid copper wire for these measurements with direct connections. Not even solder. Do you have any old coax? If you have junk coax with solid copper center that should do nicely.
Those wire wound resistors are quite stable and will give you good hobby grade standards for a very long time.Nice condition too.
It looks like a resistance bridge and you may be able to figure out the different modes by perhaps looking at something like a General Radio or Biddle Wheatstone bridge and see if you can compare the switches and connectors.
Besides being a resistance box and a bridge for measuring an unknown, these thing have other interesting modes, most of which I am not experienced enough to intelligently comment on. These modes can find a distance to fault, for example, I think they are called MUR Functions?