Forum:Claptrap Dilemma

So, I've been playing this game for well over 100 hours now (currently in the salt flats on my second playthrough; I decided to take my time with the game, if anyone should wonder about the time count), and only recently have I discovered that there's a slim chance to get an extra SDU for the backpack from the first five claptraps on the second playthrough. Well, as you can imagine, this does me no good seeing as I'm already in the salt flats (with all previous missions completed). So here's my question: Is it still possible for me to procure an extra SDU for my backpack from some of the later claptraps in the game? Or is it absolutely certain that it only works with the first five? Many thanks to anyone who can produce some answers for me regarding this.