Pole 8, rotate both upper and lower blocks by 1 mrad.
The center of rotation is the block face at x=0.
This doesn't change the gap, so the x-trajectory is not affected.
The rotation direction is from the x-axis to the y-axis.

The upper block has negative charges near the gap.
The rotation moves the negative charges at -x closer to the gap,
  and the negative chareges at +x farther from the gap.
This makes -Bx.

The lower block has plus charges near the gap.
The rotation moves the plus charges at +x closer to the gap,
  and the plus chareges at -x farther from the gap.
This makes -Bx.

y-slope before pole move: 0
y-slope after pole move: 5.5989e-006
(sign correct for -Bx)

Sensitivity:
rotating the upper and lower blocks by 1 mrad changes the slope by 5.60e-6