It is a 3GS. If you plug into Itunes, it should inform you that there is 5.0.1 fw available for upgrade and not 4.2.1.Hi.
I have the original packing material...the phone was formerly
related to AT&T...I am the first and only owner.
The Model # is: MB715LL
The Version Firmware # is: 05.13.04
The Version is: 4.0 (8A293)