is the "T" ...
This "T" indicates that radioactive material,
i.e., tritium (that's why it is a "T") is
used in this watch. Don't worry: the radiation will
definitely not be harmful to the person wearing this
watch. The "activity" used in watchmaking
is limited to a value of 25 mCi (milliCurie) which means
that no radiation will pass through glass and/or case
of this watch.
Some manufacturers make a special point of mentioning
that they do not use the maximum allowed activity and
put additional information on the dial:
"T < 25" means that the activity of the
luminescent material is below the legal limit.
If, on the other hand, you find "T 25" on
a watch, the manufacturer informs you that he has used
the highest allowed activity; this is sometimes necessary
for special purpose watches, esp. for deep-sea diving.
If you need a dial or hand replacement for such a "T"
watch, it will be necessary to send it to the factory;
handling and working with tritium requires special safety
precautions and permissions that are usually beyond
a "normal" watchmaker's possibilities. Tritium-filled
hands are likewise usually not available from watch