Here is a quote about 600EX. And a link to the quoted page.
...a 1981 catalogue in which the top of the range is one with a Shimano Dura Ace group set. I’ve never seen one of these. Second was probably the Viscount Aerospace 600ax, with the Shimano 600ax gruppo. Just below that must have been the 600ex (you can guess what that came with).
Lambert & Viscount bikes: History Part 2