One of the standard works on German civil censorship during WW2 is Horst Landsmann's
"Die Zensur von Zivilpost in Duetschland im 2. Weltkreig".At page 72, he illustrates what I think is the marking you mention.
The following image is from my translation of this section.

If this is not the marking you are looking for, please post a scan of your cover.