This is the mixture retainer pin that rapidly wears causing erratic mixture settings. The retainer clip was installed by the requirements of AD 94 25-04. The clip reduced the wear rate considerably but did not stop the wear. If the HA-6 carburetor has this clip and pin configuration, it would be a good idea to remove the pin and check for wear at least every 500 hours. Symptoms are an erratic mixture. The later carburetor bodies had a steel retainer plate that eliminated the pin and the problem. There is another photo of the improved configuration next.