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.
Since I highly doubt the unaltered Originals are anywhere on those discs, and I already own Solo and TLJ on UHD, I’m going to skip this boxed set and just buy TRoS and the Originals on UHD separately. I might add R1 and TFA on UHD later.
Also, am I the only person who hates the idea of sliding my discs in and out of cardboard? I hate sets like that.