Fritz Marti was one of the most important and eclectic inventors in matters of watchmaking. To him we owe the invention of Incabloc, of Incastar and dozens of other patents.
One of these patents is the “Preserval”, a plastic casing made to ensure the impermeability of the movement and the dial of the watch.
This is an excerpt of the Swiss and French patent.
A couple of ads in which Preserval is highlighted.
This rare Marvin from the 30s uses the Preserval
The screw on the left is to adjust the racket without opening the shell.