An underline on your dial stands for Tritium!

Tom's logic answer to your question:

Perhaps the dials were already starting to be printed with the "T" but still marked for a short while with the underline after initial printing as per prior procedure. Then they realized that they no longer needed the added underline to designate that the dial was conforming to international radioactivity standards and stopped bothering with that additional step.

Just a thought...
Best,
T.

robè per me siamo in 2 a dire che è del 63, anche secondo me è del 63. te lo dissi ieri.

poi tutto può essere