| 摘要 |
A rail including a number of slots being longitudinally formed therein and a number of options each including a guide being slidably engageable in the slots. Each slot consists of a wide portion and a narrow portion. The wide portion of each slot is proximate to the axis of the rail while the narrow portion of the same is distal from the axis of the rail. Each guide has a form being compensative to that of each slot. Thus, each option will not be radially disengaged from the rail when the guide of the option is engaged in a corresponding slot formed in the rail.
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