That side cover is from a '79 C3 or '80 C4 (large block letter badge) the side cover has been totally repainted. You can check the stock information here including the VIN and engine numbers.
www.jarlef.no/Kawasaki/Series/Z/650/Z650C.htm
A lot of times newer body work is put on older bikes, but that frame is a C model frame (rear master) so it could go as far back as 1977.
