If it was me having this problem, once I determined if one of the magnifiers was correct, then I'd use
the same target system to correctly shim or shave all the others to match. In the past I used my
good one mainly for critical work and had merely OK ones avail for garden-variety work like typical
black and white printing. Now I use only the best one and hope that my progressively arthritic fingers
don't drop it like the cheaper ones! (Knowing it was expensive does make me a lot more careful!!)
If purchasing new, I always assume correction is needed unless a test determines otherwise. The
fine focus on a 138 can slip, but there's an adjustment feature for the tension which can be checked
during maintenance intervals. For super-critical work a helical-focus lens mount could always be added, of course.