A reply arrived today from Belgium!
I had written to Prince Laurent and Princess Claire (King Philippe's brother and sister-in-law) to congratulate them on their 15th Wedding anniversary.
They replied with a typical Belgium Royal Envelope (or a typical as it can be given the many different versions). The front features a purple crown with Palais de Bruxelles/Palace of Brussels written underneath.
I love the fancy font of the address. It is something we see ocassionally from the Belgium Royals.
Inside is the typical flat card.
Translated the message reads:
Palace of Brussels
Service of His Royal Highness Prince Laurent