A locking and unlocking arrangement for a screw or nut fixing device, particularly a theft prevention screw or nut for the wheel of a vehicle, the gripping surface and the fixing piece having a specifically coded shape so that they can only be screwed or unscrewed by means of the complementary surface of a tool which matches the fixing piece. In the case of a screw, a centered or offset toothed cavity (13) is formed in the head (12) of the screw (1), an intermediate piece (3) made from a cylindrical piece (31) is combined with said screw (1) and has on one of its sides a centered or offset cylindrical cavity (32), while the other side carries a toothed cylindrical projecting portion (33) which is also centered or offset. Indexing takes place by connecting the teeth (13, 33). The related tool is made to correspond.
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