These "cracks" happend on the A310 in the early 80's. They are fractures at the 90 degree flanges where the vertical rib meets the horizontal section bolted through the skin. The 310s were repaired and had a full service life thereafter. Then titanium reinforcings were bolted to the rib skin. Very time consuming "concession" but operationally the only physical penalty was a minute reduction in fuel capacity and slight increase in weight. Don't know how they are getting away without compensation though.