Cylinder lenses are mounted in the lens banks so that they are all on the exact same axis when they are superimposed over the open apeture.
When banks are taken apart, there is a very precise operation that must be performed to assure they are put back together properly.
A simplifed decription of this process is:
The axis of each lens is determined & marked using a modified lensometer fixture.
The strong cylinder lenses are put back into the bank and each lens is aligned to the same point on the center gear.
The weak cylinder lenses are then superimposed over the strong cylinder lenses and the assembly is put back together.
The completed cylinder bank is replaced in the cylinder housing.
Using a modified projection fixture the axis of each lens is projected onto a screen to verify they are all on properly aligned.
The cylinder axis scale surrounding the open apeture is aligned to the lenses.
The turret is replaced and the lower cylinder axis scale is aligned to the correct axis.