On the aft end 26 there is a rakish sunken surface 28 that ends in a level vertical surface 30. The rakish curved surface 28 and the level vertical surface 30 are intended to get the shortened precise expansion 18 of other indistinguishable blocks (not shown). Additionally, the furthest bit of the precise sunken surface 28 ends against vertical shoulders 32 and 34. These upward shoulders 32 and 34 fit flush against vertical shoulders 22 and 24 of different blocks.