The working position II comprises a circular tray 30 in metal or plastics material, such as aluminium or PVC, is intended to receive a mould 10, and is provided with an annular flange 31 having four equally spaced apart recesses 32 each giving access to a retreat 34 in which is placed (FIG. 3) a device for blocking the opening 16 of each channel of the mould 10 facing a corresponding recess 32, wh