I checked some of my 2nd and 3rd generation pistols and here's what I found.
1. T-type marking on front trigger guard: 1006, 1066, 4566 & 4516-1
2. T-type marking on bottom of frame near magazine well: 645, 1076, 4576, 4006, 5903
3. No T-type marking found (might be under grip): 4506-1, 4516-2, 4013, 4014,
I didn't check all my 3rd generations, but IIRC, when I had the grips off of the pistols, I saw some that had the T-type marking near the bottom of the frame under the grip. It also appears the T-type marking is clearly visible on pistols made in or before 1995. So, if you look at the trigger guard and bottom of the frame of you pre-1995 pistol, you should see the T-type marking without removing the grip.
FWIW