The block has a mainly rectangular surface (16) and vertical channels (31), which widen conically in a downward direction. It has a continuous full-length profiling on one long side (11), and a matching profiling on the opposite long side (13). A long side has a continuous projection (22) in the top half and a recess (21) in the lower half, which correspond with a similar projection (23) and recess (24) in the opposite side. The top surface has V-shaped grooves (32) parallel to the long sides.
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