| 摘要 |
The device includes two regions surrounding an axis (A), the regions being axially offset and provided with X-ray positive marks. The marks (6) of the first region (1) are arranged close to the axis, and the marks (4) of the second region (2) are arranged at a larger distance from the axis than the marks of the first region. A third region may be provided with X-ray positive marks (5) which are at a larger distance from the axis than the marks of the first region, the first region lying between the second and the third region.
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