So the washer is an official Victa service fix for a wear condition in the governor. Kind of similar to Briggs' official service fix for a bowed fuel tank top with the PulsaJet carburetor: put a tiny roll pin in a convenient hole, and fit a miniature teflon washer to stop the fuel leak.
Thanks Ty, I've always thought it was just an unofficial "tenant's improvement" rework to modify the idle speed - I've seen people advocate using several washers to achieve that. In this case the carburetor has the original parts in it, so a wear problem seems to be very much on the cards, and the service fix would be a worthwhile step to take.
