WW2 Luxembourg Occupation -- a rare Zahlkarte control label: Fiscal or Phantom?

Zahlkarte Label

14 Jan 1941

Control No. 007656
40a Lux

Luxemburg P Sch A cds

1700 RM deposited

The fee for transfer of amounts from 1500 RM to 1750 RM is  
paid with the 5F Luxembourg Three Towers stamp surcharged to 80 Rpf.

During the 1900s, most European postal services offered direct cash deposit (or 'giro') accounts to their patrons.  Money could be paid into these accounts at any post office, eliminating the need for the payee to receive, present, and wait for a bank check to clear.  Today the availability of electronic money transfers and debit cards has greatly reduced the importance of these postal accounts.

In Germany, the Reichpost recorded giro account deposits on a three-part form called a Zahlkarte (in English:  a "payment card").  The postal service retained one part; another was sent to the account holder; and the depositor kept the third as a receipt.  

Up until the 1970s or 1980s, a control label was affixed to the card.  While it did not itself create the credit to the recipient's postal account, the Reichpost required use of the label as part of the administrative procedures for recording and authenticating the money transfer.  When affixed to a Zahlkarte, the control label provided the unique number that identified the transaction.  It thus served a fiscal accounting purpose and was a necessary part of the money transfer.  Accordingly, I believe it should enjoy the status of a revenue stamp, notwithstanding that it is the 80 Rpf. surcharged stamp that pays the transfer fee.

Collectors in Germany tell me that Zahlkarte control stamps are scarce and that only two are known for occupied Luxembourg, as the Reichpost routinely retained and destroyed the cards.  If you know more about these cards, I'd be happy to hear from you.  Write to Arsdorf@gmail.com .