> Later war models had a top that had a reverse thread, so to unscrew them, you had to twist the other way. This was done after the Germans were known to have found out about the existence of this type of compass.

I'm amazed by the simplicity of the solution, it's a brilliant idea. Same for the buttons sewn on the uniform, and the other everyday objects that turn into a compass, I love how they are hiding in plain sight.