amanico[JLC Moderator]
400224
Favre Leuba Bathy 50 Blue Prototype.
I knew the black dial, only, and recently discovered that a blue dial existed, but it seems it is a Prototype.
Credit: Hairspring.
The Bathy 50 is a great watch, including a mechanical depth gauge, which was quite rare at that time ( 1966 ), even if that kind of watches existed since 1964 ( Nivada Grenchen ).
Best,
Nicolas