Odd iPhone model number
I just picked up a used iPhone 5 and I noticed that it has an odd model number that I cant seem to find anywhere. I know it's a GSM black 32gb A1428, but it tells me it's number is MD640LL/A. It's not really a big deal, but I can't find that number listed anywhere.
Anyone know what this particular model is?
iPhone 5, iOS 6.1.4