Mr Kezich said he was amazed that there had not been any reference to such a script in biographies of Kubrick, who died in March. He said: "If it's a fake though, it's certainly a historic fake, dating back to the Sixties."
The attribution of the script, The Last Hundred Hours, to the late director is for now considered dubious, but for a personality as cryptic, obsessive and secretive as Kubrick's, abandoning an early work may have had a perverse appeal.
The screenplay is set in Nazi-occupied Italy where a German commando squad has a mission to blow up a bridge over the River Po during the last four days before the German surrender in 1945.
The date on the manuscript, 24 June 1964, would put the creation of the epic between Kubrick's recognised masterpieces Doctor Strangelove and 2001 A Space Odyssey.