the holding surface 2 is flat in configuration the keyway 5 1 5 2 per fig1 b has a width b 2 of around 10 mm and tapers to a width of just 5 mm terminating in a radius r of 2 5 mm the end face 2 7 u2032 can also be curved if needed or be provided with a radius of curvature r 2 so that an adapting to the shape of the lens 8 being taken up is assured the magnitude of the radius of curvature r 2 of the end face 2 7 u2032 varies in the range of 70 mm and 90 mm it is preferably oriented to the radii of curvature of the lenses 8 being secured or an average value thereof according to the sample embodiment of fig5 right side in the region of the shoulder 1 2 there is provided an undercut 1 2 a acting in the axial direction in the form of an encircling groove 1 2 a with an axial depth t 2 and an aperture angle u03b1 the groove 1 2 a is provided in the shoulder 1 2 in the region of the largest diameter d of the block piece 1 in order to assure the largest possible force of adhesion of the adhesive not shown to the block piece 1 especially when bringing up a tool against a margin of a lens being machined not shown.