oh wait just kidding my original brand new iphone 4 (the one being replaced) was a 83. but my replacement phone is for sure a 5k. and on Apple site it says all the replacements are either brand new or refurbished to original performance. So i guess you can't be sure that 5k are all refurbished unless someone buys a refurbished iphone 4 and it has a 5k number. but what it's for sure is that it seems all the replacement phones are 5k.
BTW, when u got your replacement phone, did your applecare got transferred as well? I got my replacement on Wednesday and i checked online today, the applecare still hasn't shown up on my current replacement phone. ?!