The failure would appear to be fatigue resulting from overheating during brazing of the shift levers ' boss. This would be in the thinner 0.6mm section of the tube. The downtube appears oval, so the only practical repair is replacement of the tube by a competent framebuilder experienced in fillet brazing. This is not going to be cheap and a standard tube may have to be formed to get an aero profile similar to the original.