How do you know it wasn't performed without the BATF's permission? Do they keep central database of who and when they allow a frame to be reserialized back to it's original s/n?

Of course I would walk on it without knowing the specific deets, but if the frame s/n matches the yoke s/n and the the assembly numbers on both of those match then I sincerely doubt the BATF will bring the hammer of THOR down on someone with such an example.

In my opinion (assuming the serial numbers and assembly numbers match) the correct thing to do was attempted in this instance. Whether or not it was with the BATF's guidance I do not know. For that matter it could have been done at an arsenal or unit armorer level refurb. (Most likely not the case but not entirely unheard of either.)

Obviously my opinion carries absolutely zero weight here so buyer beware.