It's just a turned brass plug to mimic the standard Aerial Rod 'F' base, with a knurled portion so you can extract it from the aerial base without pulling on the wire, and nickel plated overall.
I've got a set of those aerials so will fish them out and run a micrometer over them for you.
The only hard part will be getting them nickel plated, everything else is just standard turning on a lathe and tapping the hole for the screw.
Chris.
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