Sounds like the high speed needle(vacuum needle) seat.
It is held in place by the high speed emulsion tube. It goes in first, then when you screw the brass tube into the carb body it holds that part in place. I think it should be brass. If you post in the buying and selling section someone may have one they will sell. You might find a set of carbs that are only good for parts on ebay or this forum.
In this pic it is grouped as a set with the needle,,#2
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