Root [AT&T] Phone has wrong bootloader

[Headcase_Fargone]

Hi all,

I just picked up a refurbished AT&T Galaxy S3 (i747) and it appears to have the T-Mobile (T999) bootloader for some reason. It's definitely an i747 (sticker under battery, System About, and Info app all say it), but under Download Mode and in an app I downloaded called simply "Info" both show T999 as the bootloader version.

I've tried flashing the i747 4.1.2 bootloader (that's the version of Android currently installed) 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

The phone appears to already be unlocked. I put a T-Mobile sim in it and it not only registered but had H+ available, which is odd for the AT&T model. But I don't need to downgrade to 4.1.1 to unlock since this is the case, right?

How can I fix this bootloader issue so I can get a more recent rom like AOKP 4.4 on it?

 

[El Presidente]

Moved to the SGS3 AT&T root forum, the guys here will be in the best position to help.