I received this year a few years ago, but never wrote a post about it.
The envelope is one of the usual from the Palace, a purple crown with Palais de Bruxelles underneath. It matches the card.
Inside is a flat card, this is the same as past birthday replies from Queen Mathilde, but the message changes.
The card is in french, the translation says:
The warm wishes that you were kind enough to address to Her Majesty the Queen on the occasion of her birthday gave her great pleasure.
The Queen expresses her sincere thanks for this appreciated attention.
Included with the card was a photo of Queen Mathilde that was released for her birthday.