What is the difference between MD788LL/A and MD788LL/B?

I ordered two iPad Airs from the same vendor at the same time. I received one MD788LL/A and one MD788LL/B. I have search the forums and the internet and I find each is listed as the basic 16GB, Wi-Fi only iPad but I cannot find what is different enough to require Apple to change the part number. Does anyone else know?