He was only portrayed as being left handed in one film, Indiscreet. In other films, where a close up of Cary Grant's character writing something was required, somebody else's hand was used. In North by Northwest, and many other films, he threw stones with his left hand and punched with his left hand. Or swap with another Cary Grant fan. One way to do this is to join the Email Warbrides - the Cary Grant mailing list.
Different countries use different methods of recording and playback.
Please go to Video Standards for a complete explanation of this. If you go to the Reviews page, you'll find an alphabetical listing of all 72 of Cary's movies. I've found more. There are only 72 movies listed in Cary's 'official' filmography. He did, however, make several other appearances not listed in his filmography.