I believe the "T" actually stood for the touring suspension package that was an option available on a V-8 Regal but required with the LC2 in the turbo cars. I have never seen a n/a 6 Regal though. The GN had the same suspension but the "GN" emblems superseded the "T" emblems.
(I remembered seeing a Buick 5 car T-Type ad back in the day. There is a copy of that ad in the comment section of this article. Just scroll far enough down.)