The firmware for the AT&T SM-C105A is not on the Samsung server. This information regarding the now missing firmware is a result of conversations with Samsung T3 ATS and Samsung Experience Reps.
I have also been unable to locate the AT&T version of the firmware on any of the online archives as well.
As a result, if you trigger the AT&T "unauthorized software" lock you cannot restore the device. Recovery gets locked out, Odin will not flash an alternate firmware and unless a service center already has a copy of the AT&T firmware, they cannot help either.