Who knows? Don’t assume that this is just an instance of hasty, inadequate, or nonexistent proofreading, that an extra distorting “s” snuck in there somehow and that the “s” balance will be restored by leaving the letter off a word that normally ha one.
It’s possible that they intended to emphasize the fact that this is not just the last edition but the most last of all the lasts, never to be exceeded in its essential lastness. Course, for me to type the previous sentence, three or four overrides of the automated bane of my existence Word editor were necessary. So, maybe they were using a more primitive system, one that enables creativity of a sort without going all schoolmarm on ’em.
This is another in a long line of examples that prove Dan Quayle’s* point about the mind being a terrible thing. Without reading aloud and focusing on the letters we see printed on a page, we miss LOTS of minor errors. Minor, possibly, but the kind that can prevent a communicator from convincing a communicatee (shut up your mouth, Ms. Word) that they are in good company and should go with us where we want to take them.
*For those lucky enough to say Dan Who? there, those who think Sarah Palin was the first sign of a looming crack-up in one of our too few political parties, here’s all you need to know about him and his pertinence in this context: