| 摘要 |
a plurality of holes formed thereon and having the same number and arrangement as those of an outlet of an insertion groove and a through-hole of a lower side circular plate, the closing member having the size corresponding to the insertion groove; and an edge portion formed each in upper and lower directions on the inner peripheral portion of the holes and protruded by a predetermined height, which is inserted into an end projection which is formed the through-hole of the lower side circular plate and the outlet of the insertion groove, respectively.
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