What I've heard, and this is entirely word of mouth, is that occasionally Underworks will change the style of their binders a little bit but all of the product goes into the same bin, they don't distinguish between an old style and new, so whatever they pull out for your order is what you end up with. I checked my stash of binders (all XS)--my current ones (both white and black) resemble the one on the right of your picture, I bought both of these December 09. The one I had before that (my best guess is '07-ish) looks like the longer one on the left, another year or two before that I had one of the shorter ones (on the right).
When I used the double fronts I would occasionally notice small differences between length or the size of the arm hole, etc, but luckily it's never been an issue for me.
