**Partial Answer**

If we solve the cube with the writing on it we get:

>!<br>[![enter image description here][1]][1]

This suggest that each section is a letter in the alphabet: (I'll update the picture in a second)

>!<br>[![enter image description here][2]][2]

This is why Y and Z cards are missing, there are only 24 squares


  [1]: https://i.sstatic.net/PgccC.jpg
  [2]: https://i.sstatic.net/47aC8.jpg