Good luck getting a reliable answer to that question. I don't believe I've seen that before, and the "T" doesn't show up in the NGC verifier tool. I have a sneaking suspicion that it may have been used for a time to designate "Toned" coins, but that's merely a guess, and certainly not reliable, coming from me.
I have seen both the T-toned, and the W-white. The T coins I have seen are usually pretty spectacular. I don't think that the T/W grading lasted more than 6 months.
Before the star grade, “T” meant toned. NGC only used this designation for a couple of years, in the early 2000s.
I can see why. It was an ill-conceived idea. It doesn't take specialized expertise to know if a coin is toned.
It would have had value back in the days before the internet had pictures, but it was basically obsolete by the time it was introduced