Each Mystery Booster: Convention Edition booster box contains 24 packs from the main Mystery Booster set, plus one extra-special “R&D” card. You won’t believe the stuff they came up with!

Mystery Booster: Convention Edition is designed to emulate a Chaos Draft as it contains more cards than any Magic set ever at 1,670 reprinted cards. In addition, each pack contains 1 of 121 different playtests cards that are exclusive to Mystery Booster: Convention Edition.

Please note that the list of possible cards as well as the art from two playtest cards have been altered from the original 2019 release. Find a list of all the changes at the link below:
 

Each pack contains:

2 white commons or uncommon
2 blue commons or uncommon
2 black commons or uncommon
2 red commons or uncommon
2 green commons or uncommon
1 multicolor common or uncommon
1 artifact or land common or uncommon
1 rare or mythic rare from before Core Set 2015
1 other rare or mythic rare
1 special "R&D" card