Thanks guys.
As you will know, the Mk.V in .455 is less common and it has not been easy to find a good example for sale in Australia in recent years. With a bit of luck on my side I finally located one within my price range the other day, marked "T.C.R.E." for Telegraph Company, Royal Engineers. It is on its way north as I type this.

Looks like I will need a bigger display box!