More and more "original" cars are showing up with two different color inspection stamps. I'm begining to think that instead of night and day shift, the colors indicate the results of the inspection. Green being everything was OK and Red/orange meaning somethin needed to be fixed. I've only seen one peice of an orange "B" on my firewall as the firewall had been painted black at least once before I got it. There was a peice of sealant stuck to the firewall about the size of a quarter and when I pulled it off one day, there was a section of the "B" behind it.
The "T" on your car could be under the heater duct, since the stamps were put on by Fisher just before the rear half of the body was sent to Chevrolet, and at that point neither the front fenders or the blower motor duct was put on. It also could have been painted over if GM needed to respray the firewall due to body color overspray. Sounds like your car spent some time in the GM paint repair booth.
Check out this link for the Norwood assembly Process:
CRG Norwood Assembly Process.