I photographed this bike at the Pebbles bike show 10 years ago. The Avenger was a twin cylinder two stroke produced 1967-71. It had twin disc valves with the carbs located at the ends of the crank and behind the engine casings. The bike claimed 42bhp - more than most 500s of the time. The Avenger was replaced by the mad and bad triples.
Kawasakis of this era are very rare in the UK - I don't know if this model was ever sold here.