Hi Paul - Nikon man myself but I think you may find these links help
LINK
http://www.bobatkins.com/photography...q/DATECODE.HTM
LINK
http://www.markerink.org/WJM/HTML/eos_age.htm
LINK
http://photo.net/canon-fd-camera-forum/00M91l
That may help I hope but as for telling if it is a "genuine" Canon lens I suppose only the manufacturers can tell you - But I have to say I've not heard of any counterfeit lenses.