Green AL-KO Wear Indicator. This wear indicator is fitted to AK160, AK300, AKS1300, AKS3004 and other AL-KO coupling heads. AL-KO part number 603655 The wear indicator is fitted to the front of the coupling handle and the red wear indicator is fitted to it. Please Note on some heads you need to take a nick out of the back edge to slide it on and glue it to the head. 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 U423