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This is another member of the Average Dice family.
You have to make the average of 3 faces of the inner cube (sharing a common vertex pointing upside) to find the result of the D8.
For instance, in the image, as 1+7+13=21 and 21/3=7 the result would be 7.
More informations here: https://www.shapeways.com/forum/index.php?t=msg&th=3847