Here are a couple of examples. Unfortunately I don't have a really good close-up of one, but it wouldn't be hard to arrange if required.
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This is the one on my car. It is not an original NASCO job, but an original period style aerial. Like Ken said, the last section is fixed. This one is pretty big, with the last section probably 7/16" in diameter. Note also the tall bakelite base. The tip is a large red ball, possibly made of bakelite.[/box]
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This is Graham's which is a NASCO red-tip job.[/box]
Both these aerials require a CB radio style cable fitting, in fact one of these can be used if you don't have the original by cutting the male pin off (ouch) close to flush with the insulator.
Hope that helps.
cheers
RET