T-Mobile branded S3 somehow became AT&T.

[BlueJellyfish]

When I received my S3, it was on a T-Mobile branded stock firmware 4.1.1. I wanted to try CyanogenMod, so I installed it, but didn't like it.
Here's where my problem begins. Upon wiping my phone back to stock to get rid of CyanogenMod, I find that it's now AT&T branded, despite the physical phone being a T-Mobile T999.
I tried rooting it and using CSC changers, but those tell me that the PreconfigUI is SingleCSC, and thus apparently unchangeable.
What I want to do is somehow get my phone back to T-Mobile branded stock.

P.S. I do not have a MicroSIM yet.

 

[tube517]

Goto settings and see what it says under "model"

Settings> About Phone>Model number

Did you buy this used? Where did you buy it? Are you seeing an AT&T Splash screen?

Also, what do you mean by "It didn't like it" as far as installing Cyanogen? Need more specifics