Red Al-ko wear indicator. AL-KO part number 603654 The wear indicator is fitted to the front of the coupling handle and fitted to models AK160, AK300, AKS1300, AKS3004 and other Al-ko coupling heads. If the wear indicator is green your towball is within the permitted range. If the indicator is red the towball is on it's lowest limit ( 49mm ) or less. Ref U422