At IPP 2008 in Prag, Russian puzzle designer Dr. Vladimir Krasnoukhov gave birth to a mechanical puzzle called BRICKS (please do not mix up with other puzzles from him under the name of SERIOUS BRICKS and BRICK PUZZLE --> those are different). As per my internet investigation this puzzle is a rare one nowadays and not commercially available any more. Who else than my puzzle friend Bernhard Schweitzer had a spare copy for me which changed hands for 20 Euros which was the price Krasnoukhov sold it in Prag.
BRICKS is your typical 3D packing puzzle which comes with 9 pieces and a tray. Of course the ultimate goal is to place all pieces inside the tray so that nothing stands out of the tray (flat surface). I could read that this very puzzle has the same structure than a puzzle from Conway/Slothouber-Graatsma but scaled by 1,5x and 2x in two dimensions. Consequently this distortion can lead to more false solutions making it more perplexing than the original. My copy arrived in the solved state so I blindly emptied the tray and scrambled the pieces. After my girlfriend went to bed a few hours later I tried to solve it. Surprisingly it only took me less than 10 minutes to solve (a day later on my second try I only needed 3-4 mintues). Somehow it seems really logical how it has to be built in. My difficulty rating can therefore not be higher than a 4 out of 10 based on the experience made.
Since this puzzle is not widely available you have to look for puzzle auctions and have to be lucky to get your hands on this puzzle. On the other side I believe it is not overly difficult to build your own copy (besides the tray may be). I think normal puzzlers will not need to have this puzzle but for serious collectors it is inevitable to look around for it.