Actually that part IS plastic.

It's the Delrin shoulder washers used for isolating the connectors and their diameter is virtually the same as the diameter of the through holes that the connectors are protruding through.

I could have just mounted the RCAs without them but the diameter of the shoulder on the shoulder washer was 3/8" and the diameter of the threads on the RCA were some metric dimension so I just made it simple and drilled 3/8" holes.

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