Buddy Jeff..as mentioned earlier by Orchi..(+)

Err...due to regulatory requirement by the authorities...
for the use of Tritium to replace the hazardous stronger...
radioactive Radium(in tiny minute amount) as the fusing agent
to ignite Phosphorous...that were mixed with paint & boding compound...
at the time(early 60s)...to make the mixed compound luminous in the dark...
the "underline" was added to "SWISS" Dials that were remaining
in stocks...or those Dials that were on Dial's manufacturer's stocks...
to indicate...the usage of Tritium...as there was no clear
guidelines by the authority on any type of markings to be used...then.


Soon later...single "T" plot on each sides of the "SWISS" markings...to mark
the usage of Tritium...
were added on some Dials in stocks that already had the "underline" marking.

Soon after that...it was compulsory to have "T" marked on Dials...which later
indicated compulsorily...the amount of radioactivity of "< 25"... <br>
So there were also some Dials...even marked with added "SWISS-T<25" with "SWISS"... <br> or "T SWISS < 25"...may be. <br>
It was earlier thought by many collectors or even in published materials...
that the use of Tritium...began from 1960...that may NOT be true anymore...

There are evidence pointing to the use of Tritium(fusing agent in tiny smallest
quantity to ignite Phosphorous) was mixed to the paint compound as luminous material...even as earlier since...1958.

Oh btw...Orchi was recently told also...that ROLEX in its entire history of
producing ALL watches with Tritium marked Dials & Hands...had only consumed
a very small amount of about ONE KILOGRAM of Tritium...ONLY.

Believe it or not...

Best regards
Orchi.