could be AJS or Matchless lightweight, wich appear in 1958 It is more like this one http://motorbike-search-engine.co.uk/classic_bikes/matchless-g3s-mercury.jpg which is a G3 rather than the lightweight G2 (or AJS 14) which had unit construction. http://motorbike-search-engine.co.uk/classic_bikes/1958-MATCHLESS-G2.jpg
In 1958 they still had the pushrod tubes exposed http://motorbike-search-engine.co.uk/classic_bikes/1958_ajs_350.jpg
You could get a rigid frame on these until 1955 but this one is no later than 1951 because of the gearbox type.
It has to be 1967 because it has the new tank badge but still has the small front brake. I can't see enough of it to tell if it is a 500 or 650.