Prime Boxes of (51)                    Torsten, 1994
                      Update: 7x8x20 - Helmut Postl, Jan 1998
    1
  1 2 1

  4xNx 13?
  5xNx 11?
  6xNx 7?


The first box found by Helmut Postl was 7x8x20.


Impossible:
  NxNx{2,3}
  4xNx{4..12}
  5xNx{4..10}
  6xNx{4..6}
  no odd box  (checkerboard 4:1)

Ref:

[1] Helmut Postl,
    A 7x8x20 Box found.
    Letter to T. Sillke dated 26 Jan. 1998.