Designed by Rick Eason 2001, made by John Devost 2007.
(Bocote and Cherry, 4 pieces, 2.4 inches square;
as described on Rick Eason's Page, this puzzle was also mass produced as
"Confusion Puzzle Mental Block" shown on the right above - wood 2.9" square)

Four pieces formed from unit cubes and rods of dimensions 1/2 by 1/2 by 2.5 units.
The photos below show the four pieces positioned to be assembled, and pairs put together; the final step slides these two halves together to get the solved cube shown above.



Further Reading
Rick Eason's Page, from: http://www.mechanicalpuzzles.org/puzzles/index.html
Amazon: http://www.amazon.co.uk/Lagoon-CONFUSION-PUZZLE-MENTAL-BLOCK/dp/B004KYNB76