I've been curious with T-3s for a while now. I recently acquired a set, and my '69 still had one remaining, but they are different. Not in the side, not in having vs not having a triangle, but in the texture/design within the triangle.
I will have to look at them again just to confirm, but IIRC, two of the blubs have a "lined" texture that run from top to bottom; where as the other bulb has a semi-rough looking texture within the triangle area.