The insertion element (21) has fastening pins (21c) e.g. threaded pins, symmetrically arranged with respect to a middle plane (PM) of a cylindrical body (21b). The cylindrical body defines two retainers (21f, 21g) e.g. blind holes, which have identical diameters. Openings of the retainers are discharged at end surfaces (F1, F2) of the cylindrical body. An axial body (21a) is provided with an end in which one of the retainers is fitted and another end, which forms rotational bolts that elongates the cylindrical body.
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