i think it would be pretty easy to guess all the combinations. Just make your way across the keyboard and spam the backspace key. I think the yellow box makes sense on this one.
I remembered that ZA was South Africa, but I didn't remember why. South doesn't begin with a 'z' in Afrikaans, my first guess. And not in any native language, seemingly.
From Wikipedia: Upon formation, the country was named the Union of South Africa in English and Unie van Zuid-Afrika in Dutch.... So as I assumed, the Dutch name of the country starts with a Z. But to be honest, I'm too tired and lazy to check further if this is the real reason for the country code.