[Station X]

I've also been looking at the diagram and have come to much the same conclusion.

With the meter switch set to position 2 it should be possible to measure the negative bias voltage directly.

Regarding the meter "pegging" in position 5, it appear that there are diodes D603, D604 and D607 which will only conduct if the grid voltage goes positive. This should prevent the meter from pegging when the grid voltage is in its normal negative state. I haven't looked at this in much detail though.