Hi remontang,
Welcome to Malaysia Watch Forum.
Back in the day Rolex, like most of the other watch manufacturers, used radioactive Tritium as the Luminescent material on the watch dials. These watches came with the T<25 designation on the dials.
After 1998 Rolex switched to the modern non radioactive Luminova to provide the night glow on the watch dials. Hence we no longer see T<25 label on contemporary Rolex timepieces.
Hope that answers your question.
