I believe that I may know who created this, if it is a.22 then careful inspection of the barrel may show it to be sleeved with Parker-Hale parker rifling insert. If this is the case, then it will have originated from the one and only Les Westley. Before legislation became too restrictive, he was converting old rook rifles with said insert to CO2.
So a very good chance it's one of his, though the one I saw was marked with his name.
HTH