Not only the piece is unique but the 6x6 square solutions seem to be unique, too.

To continue the puzzle: remove one piece so that the remaining pieces cannot make a 9x4 rectangle. The piece in this case is not unique.

Juha Hyvönen

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There are six solutions possible with the seven hexominoes:


Only the L-hexomino is present in all solutions, so this piece has to be removed.

Aad van de Wetering

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The piece is "L".(see attached file)

-- Koshi Arai