Here is one on a DDR cover that is a little "out of the ordinary."
The cover is a mailing from VEB Minol in Rostock to Ostsee-Druckerei in Stralsund.. The large ZKD in the return address tells you that VEB Minol, a DDR state-owned gasoline and oil reseller, was a member of the Zentral Kurierdienst, a separate courier/mail service for government agencies and state-owned businesses. Michel catalog lists the ZKD stamps/mailing labels (not used on this cover) in its Deutschland-Spezial
catalog. They date from 1955-65.
Ostsee-Druckerei, a printing firm, may have been a state-owned business but it was not a member/qualified user of ZKD. So, the label indicates this. It must not have been an unusual situation, because the label has the identifying code "ZKD 7" .
Specifically, the label states, "Handing over as normal mail - Recipient is not a ZKD participant. Fee paid. ZKD control."