* I don't know what the first box is, but it is very nice
* Collector Booster featuring a beholder
* Unknown female character (for me) on the prerelease pack
* Strixhaven has an extra helping of extended-art cards with 69 extended-art rares and 12 extended-art mythic rares.
* In addition, [...] all 70 rares and 10 mythic rares from the Commander (2021 Edition) decks are available in an extended-art version (non-foil only).
Also
* The Commander decks are said to feature 81 new cards
#298-301 seem to be common reprints in the Starter Kit, just like Knight of the Keep and Goblin Assailant#299 is the pro tour play promo Gandalf the White
#300 is the store championship promo Saruman of Many Colors
#341-349 have the Art Deco Treatment
Skyscrapers start with #350
#450-460 are the Gala Greeters box toppers
Buy-a-Box = #461, Bundle promo = #462, Dark Frame promos = #463-467
* Collector Booster featuring a beholder
* Unknown female character (for me) on the prerelease pack
* 3(!) Set Boosters, featuring unknown (for me), mind flayer and a blue dragon
2) Shocklands (leaked)
3) Owls?
4) x-spells?
(previewed)
Edit: found it. They are numbered as C21 cards but are only found in the Strixhaven Collector boosters
https://magic.wizards.com/en/articles/archive/feature/collecting-strixhaven-school-mages-2021-03-25
* Strixhaven has an extra helping of extended-art cards with 69 extended-art rares and 12 extended-art mythic rares.
* In addition, [...] all 70 rares and 10 mythic rares from the Commander (2021 Edition) decks are available in an extended-art version (non-foil only).
Also
* The Commander decks are said to feature 81 new cards
You'll be glad to know that release and spoilers have been postponed for two weeks.