Ok I'll look for a BST plug with an o-ring.
Just curious, is this an aftermarket radiator you think or did some discos have this smaller fill plug?

 

No Land Rover I am aware of has a radiator like that, especially a nineties NAS Discovery. Aftermarket

 

The radiator that metzdmichael has is a Nissen's 64029 with metal tanks.
The fill plug came both in plastic or brass, and it was smaller than the OE plug # ERR4686.
IDK if Nissen's still makes the metal tank version, now the 64029 comes with plastic tanks.