Having a closer look at the boards, I've noticed that the resistors are all on the various inputs, there are two more carbon composition resistors between the front binding posts and hybrid (for other functions and ranges) and also on the trigger input from the respective BNC on the rear panel. There is also one across the control data lines to the optional relay card too, so I think it is likely that they were selected for surge rating to handle any transients that may come in.
The True RMS converter board also has a few, but two are 100Meg resistors, which I believe would have been chosen due to production limitations of the time making such high value resistors unavailable in 1/4 watt size metal and carbon film resistors.
As for calibration, I assumed this would be able to get done as the other models are still serviced by Keysight. For the 3457A, they just say to contact them, so I will have to do so and see what they say....
Jared
Jared Cabot