But I will be less certain that the underline means that it is a sign of transition from radium to tritium...
There is one thing which is certain, which is that in 1961 the reglementation required that tritium was used instead of radium.
From which year yours is?
1963? 1962? There are a lot of theories about the underline meaning.
And till now, nobody can say, for sure, that it has a certain meaning.
Same with the Exclamation Dot, by the way...
Back to your watch, now, this is a superb example you show here. The patina, the condition are greatissime, and this underline is the cherry on the cake.
Bravo, Amico mio.
Nicolas