What is this marking on a German truck?

Solution 1:

During WWII, the Germans devised a system of tactical symbols.

This picture details what these symbols mean:

I believe it's number 16 in the picture, which meant armored car.

_(Source)

Additional Information: I've found a large PDF that you can download and look at. It is a manual that was constructed by the United States as well as Britain that depicts and defines what all these symbols mean. On page 26 of the PDF, you can see the close symbol again. The German word for this vehicle is Panzerspähwagen which according to Google Translate, is Armored Car. The United States referred to these cars as Scout Cars, while Britain referred to them as Armd C.