A plate (1) includes blocking elements (4b, 5, 6) including:at least one slot (5) comprising a first portion opening into the hole (2) and a second portion extending over a section of this hole (2), at a distance from the edge defining this hole (2), this slot (5) thereby isolating a portion of material (6) this portion (6) has a first part (6a) with a radially increased thickness and a second part (6b) having a radially reduced thickness this part (6b) has a thickness such that the portion (6) may be radially mobile between a withdrawn position and an active positiona pad (4b) engaged in the slot (5), able to be moved in such a way as to bring the portion of material (6) from its withdrawn position to its active position.
