Here’s a HD picture where you can definitely make out what the discs are, and even what the letter from Mark Hamill says. (You guessed 100% correctly as to what they were)
https://pisces.bbystatic.com/image2/BestBuy_US/images/products/6393/6393929_sa.jpg
So, that’s one mystery down: It’s absolutely one bonus disc per movie, like many expected.