Attack Piece Block
From the Super Mario Wiki
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Attack Piece Blocks are blocks that appear in Mario & Luigi: Bowser's Inside Story, Mario & Luigi: Dream Team, and Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey with the letter "A" on them. They resemble pieces of a puzzle joined together to make a block.
Mario & Luigi: Bowser's Inside Story / Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey
In Mario & Luigi: Bowser's Inside Story, Attack Piece Blocks are scattered throughout Bowser's body and in places outside Bowser such as Dimble Wood and Peach's Castle. Blocks inside Bowser's body look two-dimensional, while blocks in the outside world look three-dimensional. There are ten Attack Piece Blocks in almost every area; they will give Mario and Luigi an Attack Piece if struck. Hitting all ten will give the bros. a new Special / Bros. Attack. Sometimes Mario and Luigi can find several of these blocks all at once. Once in a while they can get all ten at once.
They return in the game's Nintendo 3DS remake, Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey, found in the same locations as in the original game.
Mario & Luigi: Dream Team
Attack Piece Blocks return in Mario & Luigi: Dream Team, with a new design, but still giving Attack Pieces to the Mario Bros. when hit. These Attack Pieces, instead of producing Special Attacks, produce Bros. Attacks when all ten are collected, exclusive to Mario or Luigi in the Real World. They also also produce Luiginary Attacks for Mario and Dreamy Luigi in the Dream World when all ten Attack Pieces are acquired there from Attack Piece Blocks.