Thats a 'fill tower' standard stuff on 3.9 RRCs 1990 onwards for a short while then replace with a straight pipe that the 3.9 discos have now, I didn't know that 3.5 discos had one fitted.
The plug or the fill tower isn't shown in either my RRC or disco parts caralogues.
The plug looks very much like the standard rad top plug (was NTC4609 now ERR4686) which has a 1/2" bsp thread.
If hopefully it's a BSP thread then I would go to a plumbing shop ie. B&Q where I got my 'just in case' 1/2" BSP plug and look see... next size up or down to suit.