The pentacubes with their K"unzel numbers.

     x  :  a cube
     y  :  a cube on the second layer
     z  :  a pile of two cubes


   x
   x       x       x
   x       x       x       x
   x       x       x x     x
   x       x x     x       x x x

   10      11      12      13

   x x       z         z
     x       x         x
     x x     x x     x x

   20         21     22

   x x       z         z       x         x       y         y
     x x     x x     x x       z x     x z       z x     x z       z x
       x       x     x           x     x           x     x           z
   
   30         31     32         33     34         35     36        37

   x
   x         x         x
   x x       x         x
     x       x z     z x

   40         41     42

     x       x
   x x x     z x
     x       x

   50        51

   x
   x x       x x
   x x       z x

   60        61

   x x       x x     x x
     x x       z     z
     x         x     x

   70         71     72

   x x x     z x
     x       x       x x x
     x       x         z

   80        81      82

   x x
     x
   x x

   90
 
The used notation has the advantage over the ascanding numbering, to
group the pentacubes. It is derived from their form.
The pieces 10, 20, 30, 40, 60, and 70 are easy to recognize, as the
form of the piece has similarities with the first digit. The no. 50
reminds at the 5 points on the dice. 

Reference:

- Ekkehard K"unzell
  Spiele mit Pentakuben (games with pentacubes)
  http://www.uhlgmbh.de/resbuch.html