Couple possibilities. One is that the inscription is a forgery (King is a very desirable signature and very rarely signs, so it's a popular target for forgers, and forgers will often do exactly this - sign inferior quality copies of the book like a BCE and then leave some little hint like a date that predates publication because, weirdly, many of them actually don't want to fool experts. They're trying to trick lay collectors into buying a fake, but many of them are actually in the book trade and will deliberately leave clues that the signature is fake in order to prevent their forgeries from negatively impacting the historical preservation of rare books in the future). A second possibility (though much less likely in my opinion) is that it was a copy given personally to a friend of King's before publication. This seems very unlikely to me though, since there's no way he would have given his friend a book club edition instead of a proper first edition.