May not mean anything, but I found a wheelweight on the floor of a repair shop with a "T" and "1.75 oz" on it.
With severe effort, clipped it open with diagonal cutters.
The plastic is thick and the shiny core could barely be nicked. And it is magnetic. So, steel.
Over at https://thewheelweights.com/clip-on-...d-lead-weights they state, "T wheel weights are used for light-weight truck rims such as Dodge and GMC. Our T weights are gray coated and comes in boxes of 25 pieces each."
Their wheelweights are lead. This one is steel.
So, apparently "T" is a style/shape, not a composition of its materials.