Stuck with a possibly bad bootloader topic

Hi all,

I posted this in the S3 specific forum but not getting a lot of traction there. Just picked up a refurbished AT&T Galaxy S3 (i747) and it appears to have the T-Mobile (T999) bootloader for some reason.

Here's what I've found model and version-wise...

System settings - About device:
Model number SGH-I747
Android version 4.1.2 (stock Touchwiz)
Build number I747ZSEMC1

Info app from Play store:
Boot loader version T999UVDMD5
Finger print samsung/d2vl/d2can:4.1.1/JRO03l/I747ZSEMC:user/release-keys
Product d2vl
Model SGH-I747

Download Mode shows T999 bootloader.

Putting a T-Mobile sim in registers on the network and for some reason I get H+, which I didn't think was possible with the I747 AT&T model. Don't have an AT&T sim to test with at the moment.

I've tried flashing the I747 4.1.2 bootloader but I get different error messages depending on which recovery I use to flash it.

Stock recovery:

Verifying update package...
E:signature verification failed
Installation aborted

Clockworkmod:

E:Error in (file path and name)
(Status 7)
Installation aborted.

TWRP:

E:Error executing updater binary in zip (file path and name)
Updating partition details...
Failed


I've also tried using the Cyanogenmod installer under Windows and it says that it installed successfully but Touchwiz is still there. I noticed a red triangle during the process as well so I suspected it wasn't working.

My primary concern right now is how can I fix this bootloader issue so I can get a more recent rom on it? Any suggestions?