Model number of iPhone 4S Officially Unlocked
The model number doesn't have anything to do with whether the phone is locked or not. The model number is tied to the unit's hardware components. All current black and white iPhone 4S' have the model number: A1387 

The only time you'll see a different model number during the same production line is when Apple makes a hardware change (such as the iPhone 4 GSM and CDMA versions, or the 3GS "old" and "new" bootroms).

More details found over at Apple.