I had a VERY complicated answer where I used a Text input and a Union to create a data row, then pivoted into column to create the letter string.
Then I checked someone's spoiler and remembered the Field Info tool. I had never used it before.
Now other than the Formulas (Which I showed in the screenshot), it's pretty simple.
This was a creative and different challenge!
Would just like to point out that I was trying to work on this challenge yesterday at the Solutions Center at Inspire, but had soooo many people come to ask questions that I could make no headway. Best distraction ever!
(some of these are guesses... thanks for livening up my afternoon!)