As i understand it, the letters stand for the name of the weights themselves, i.e. company names. TAW stands for Tape A Weight, and other letters represent other companies. The exception to this 'rule' is a Z or Zn, that denotes Zinc material weights and Fe is some alloy that dont melt, not in my pot anyhow, and i have only found these on Corvette and pontiac wheels ( so far ), but they float.
for the most part i have been ignoring the letters, i drop the weights from table top height onto the cement floor, if they thud and dont bounce they are lead, if they ring out or bounce they get set aside, along with any weights that are riveted to the clip. so far so good.