I remember seeing them back in 1972, the first time I went over there. Autobahns are posted, always have been. It is 130kph (about 80 mph) I was wrong on that, oops.

Quick google search came up with this:The German Way & More | The German Autobahn - Driving in Germany - Autobahn Sticker/Vignette

One other word of caution is be aware that German lawmakers have decided photo enforcing cameras are legal and they use them on the autobahns. Mostly in the city but they can be anywhere. Next to your picture driving the car is your speed at the time of the picture and they cannot be challenged in court.