inner wall 5 of the distance 16 in fig1 to 5 has become the base in fig1 fig6 and 7 show a refinement of the spacer 16 shown in fig1 to 5 the variation consists in that the base 2 as well as the inner wall 5 are provided continuously with grooves 48 which extend at right angle to the flat intermediate regions 3 a and 4 a of the flanks keep a distance from the flanks 3 4 are all identical and equidistant to one another these grooves 48 may be formed by embossing they facilitate the bending or the folding of corners of the spacer 16 for this advantage it is preferred to provide the grooves 48 they are well suited for all embodiments of the present invention the embodiment illustrated in fig8 to 12 differs from the embodiment illustrated in fig1 to 5 only in the form of the interspaces 50 which adjoin the base 2 of the spacer 16 while in the example of fig1 to 5 the interspaces 50 steadily increase starting from the flat intermediate regions 3 a and 4 a up to the base 2 they increase steadily in the embodiment of fig8 to 12 starting from the base 2 up to the flat intermediate areas 3 a and 4 a which causes an undercut seen from the base 2 this undercut ends at a wall.