A Japanese Pokémon 151 booster box is a sealed box containing 20 packs of 7 cards each, celebrating the original 151 Pokémon with new artwork. It is a special set from the Scarlet & Violet era, known for its nostalgic appeal, unique illustrations, and potential for rare pulls, such as Master Ball reverse holos and special illustration rares (ARs). 
Contents and features
Booster Packs: Each box contains 20 booster packs.
Cards per Pack: There are 7 cards in each pack.
Card Artwork: The set features updated artwork for all 151 original Kanto Pokémon, including full-art cards of fan favorites like Charizard, Blastoise, and Venusaur.
Nostalgic Appeal: This set is particularly popular with returning players, as it focuses on the original Pokémon. 
Rare pulls
Master Ball Cards: You have a chance to pull Master Ball reverse holo cards, with one guaranteed in each box.
Illustration Rares (ARs): This set has a higher chance of pulling ARs compared to English versions.
Secret Rares (SARs): The set includes secret rare cards, and there is a possibility of getting multiple in one box, with some, like the Charizard SAR, being highly sought after.
"God Packs": Some Japanese 151 boxes can contain "God Packs," which include a full evolutionary line of a starter Pokémon (e.g., Bulbasaur, Ivysaur, and Venusaur). 
Important details
Language: The cards are in Japanese.
Set: This is the Scarlet & Violet 151 expansion, coded as SV2a.
Age Recommendation: Recommended for ages 9 and up.