I received this reply this week from Belgium. I had written to King Albert and Queen Paola in December to wish them a Merry Christmas.
The envelope they used is #3d.
Their reply is a flat card with a typed message and printed on signatures. The card's message written in French, Dutch, German and English
Belvédère at the top is the name of their residence. 
The card says:
Best Wishes of the Year 2024
Albert
Paola
One things that is interesting is the decoration on the card have a watermark from iStock.
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