Hi...I just measured my Mac pro laptop and it measures the same as the dimensions you gave and I have ordered 4 of these shells to put on our Macs. They fit like a glove and snap on easily, look good and protect.
Hi! My Macbook Pro is the June 2012 edition and I believe that the 13B42 is the 2011 edition, yes? I think the best thing to do is check the dimensions of your 13B42 and compare it to the dimensions of this snap on cover. That's my best answer for you. I hope it helps!