I just posted a scan at "Photos" of a card I found two weeks ago at a
local stamp and coin show.

It's a postcard sent from Monschau post office by Vennbahn officials
to a German at a Monschau address on the German east side of the
Vennbahn tracks, stating that a freight package arrived. By
returning the card, instructions say, the package would be
delivered. The card shows receipt for the item in Mutzenich exclave
on the west side of the tracks.

Notice the card is in German, but inscribed with the Belgian
printer's name and address down the left side. Even though German is
the language of the railway by treaty, note the French language
spelling for Monschau station "Montjoie" stamped on the reverse plus
the typical French language Belgian railway packet postmark at the LL
on the reverse. Coupled with the German stamp used at Monschau's
German post office to mail it, it's an interesting multicultural
affair.

I have another like the pictured one, but mailed at Monschau to a
recipient at the town of Hoefen, which I've not been able to find on
a map.

Regards
LN