identify this part please

[Ur5by9]

can anyone tell me what this brass part is called.

I'm looking to modify the radials on a Diamond v2000 antenna by extending the length of the radials.

The guide I am following shows this brass part which I'm unable to identify

many thanks in advance.

Image taken from another website

[Blackolive]

knurled expansion insert

http://www.theinsertcompany.com/brass_k ... lastic.php

[Ur5by9]

Looks close enough

Many thanks