I also thought the official story was that "EpIV ANH" was added in '81, but I've also heard people say that LFL doesn't know its own history and the alteration was in '79. A story I've heard that supports '81 as the date mentions how surprised people were to read "Episode V The Empire Strikes Back" in May of 1980, but then again they just might not have seen the '79 re-release if that's indeed when it was added.