Looking on the Dingocraft website it's .500" dia. X 5.1" long with a spigot turned on one end. I would guess that the spigot length is around 5/8"-3/4", obviously the spigot dia. is important, you could use an Imperial drill shank as a plug gauge to determine the diameter.
Silver Steel bar is precision ground and comes in 13" lengths, 1/2" (.500") dia. it is a standard stock size and you can get it from any good Engineers Merchants or even from RS Components, pack of 3, part no.680-202, £12.35 per pack + VAT