Dunstall closed down in 1982 so they're certainly not made by them. I can't recall them ever making that sort of shape silencers though. They appear to be opened again as on this link:
http://dunstallmotorcycles.com/ the site doesn't appear to work at all though...
Strange, they look more like Norman Hyde or Triumphs own TOGAs. I seem to remember that Dunstall Patent silencers looked more like long tapering megaphones with 7 small outlet pipes on the end, this sort of thing: