A regular pentomino (all 12 pieces built of 5 squares) in a square puzzel surface with a hole in the middle. According to Tenyo there exist no more than 65 different solutions for this puzzle.
My free download Pentomino solver (for Windows) finds 65 solutions as well.
The square hole in the middle could also be placed at 9 other places, this results to much more solutions.

All solutions displayed

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