Two letters, two jobs
The top letter and the bottom letter never describe the same thing. Cover one with a finger, solve the other, then swap — and prove each rule against a box that does not have that letter.
Codes
2 posts on this. Written by Mat and Alicia, who prepare pupils for the Buckinghamshire Transfer Test every week in Winslow.
Repeats first: cracking a vertical code
Find a letter that appears twice, ask what those two figures have that the others don't, and the letter has told you what it means. One position at a time, and check every letter before you commit.