It's a reproduction (because there's only one known original copy and that's in a Danish museum) of a set of Belgian cards produced in Brussels between 1770 and 1786. They're regular cards, and you can play regular games with them, but they each have a short musical score on them. The composer (or composers) of the pieces are unknown, but they seem to be present as an early form of, well, jusdge for yourself.
The thing is, the pieces pair up, melody and harmony. The two black aces are one pair, as are the black twos, black threes and so on, all the way up to the red kings. The red cards are in G major, the black cards are in D major — except the queens, which are in G minor and D minor. This means that if you're playing a game and want to tell someone what your cards are, you can simply whistle or hum the tune and the person with the matching card will know which one it is; everyone else will only know what colour it is.
On the one hand, this looks to be a form of cheating. On the other, it looks to be a way to incentivise people to learn to read music.
Yes, a very interesting deck. Thanks, Carlos and Jesper!