A very simple mechanical calculator of the Troncet-type, more commonly known here as 'an addiator'. This Produx was probably the main competitor to the Addiator.
This particular little calculator likely dates to the 1950-ies. The protective sleeve is made of a plastic (PVC or vinyl) and it is made in Germany - West.
It is of very simple and low-cost construction. The Produx is quite compact at 4½ by 2¼ inch and very thin. The stylus made of rolled-up brass is a bit too flimsy; the tip is often broken off as it is in this case. A toothpick inserted in the stylus makes it usable again. Both adding and subtracting are on the front of the calculator, making it a bit more convenient to use than the original Addiator.
The sliders protrude through the bottom of the device, so resetting to zero is a simple pushing back - set it on the desk and push down.
These take a bit of getting used to, but then work surprisingly well :)