as the upper and lower faces of the supporting body holes 15 are provided in the rib and arranged symmetrically in correspondence with the holes 8 at the front face 9 the arrangement of the holes 15 allows to place and fasten and also to lock the supporting body 7 on the locking base 16 of the niche assembly the locking base 16 is rectangular and has sides which are identical to the sides of the back face of the supporting body 7 it is provided with holes 17 for bolts screws or the like for locking it against a surface where the niche assembly is to be placed the locking base is also provided with cylindrical emerging pins 18 similar to the pins 6 of the box 1 the pins 18 are placed so that they coincide with the holes 15 of the supporting body 7 therefore the supporting body can be locked on the locking base 16 by introducing the holes on the corresponding pins 18 finally the locking base 16 has a retaining means which acts as a locking device the returning means include a rectangular plate 19 which is locked at the upper part of the locking base 16 and on the face which.