I just checked the movement for linearity, it seems fairly linear, certainly not as bad as the instrument as a whole.
It is a 30 micro amp movement. I put a 1 M ohm resistance and my Bremen BM 869S in series with the movement, then connected this to my power supply.
This is what I found:
Current μA Readout
0,00 0,0
4,92 5,0
9,96 10,0
15,00 15,0
20,00 20,0
25,05 25,0
30,10 30,0