Cortebert wrist watch

[Sontacchi]

I recently found this watch.
On dial is writing:
CORTEBERT
ANTIMAGNETIC
SWISS MADE

On the side:
PLAQUE G20 SC

On the backside:
GRAND PRIX
ANTIMAGNETIC
SWISS MADE
FOND ACIER INOXYDABLE
9433

On the movement:
17 JEWELS
SWISS MADE
CORTEBERT *** - three stars
CAL.678.

How old is this watch and wath it mean:
"plaque G20 SC" and "CAL.678."
Anything about producer Cortebert?
Thanks.