The vintage VW dash mileage counter has a magnetic base and the size is about 4x3cm.
It is an accessory that you can use to note the mileage of the next fuel fill up or oil change.
After the tank is filled with fuel, you just roll the wheels on the counter to match with the speedometer mileage and you are on the way. You then compare your mileage to it and you know how far you have driven. After about 350 kilometer with your VW bug, it will remind you to fill fuel up again. It is a must have in a vintage car without a fuel gauge. No more notes...
You can call it vintage dash mileage counter or:
vintage mileage counter
KM kilometer counter
oil change reminder
small mileage counter
manual dash remind counter
dash trip counter
The first picture shows the Vintage dash mileage counter in an German Autozubehoer Katalog Halt from 1965.
Below you see lots of variations of the miles counter sold in in German between 1953-1970.