Sorry, I guess those are part numbers.
From a quick search it looks like a MC137LL is a 32GB iPhone 3GS with a black case.
My iPhone says MB717LL in that field in mine is a 32GB black 3GS also.
Edit: You bought your phone direct from AT&T, right? Looks like the MC137 number is for AT&T phones and MB717 is for phones purchased from Apple. (I got mine at an Apple store.)
Wow. I had no idea it was this complex.
But, it looks like you got a legit 3GS.