Flyboy Dave
The thrust washer has nothing to do with a seal it is used to keep the aluminim prop washer from contacting the aluminum case and gauling it when using an electric starter. If no one used a starter where they pushed back on the shaft the steel washer would not be needed for separation.
Some of the GP 42 carbs tend to leak around the plastic fuel nipple and that is why they changed to an aluminum one with a soft aluminum washer (gasket) on each side. Maybe the carb was a warranty return and faulty that they sent. It was mentioned that it wasn't new.