A first piece mounted in a fixed and rigid manner on a rigid annular collar of a second piece having an end opening, includes a transversal wall; an annular wall forming a skirt, whose inner face includes a part having a larger diameter and a part having a smaller diameter; a holding toric body adjacent to the inner face, deformable between a retracted state and an expanded state, and filled with a swelling medium; and at least one inlet/outlet port for the swelling media, communicating with the holding body. The first piece is dimensioned in relation to the collar, so as to enable the first piece to be slipped onto, or removed from, the second piece, when the holding body is in the retracted state, and to be mounted onto the collar in a fixed, rigid and tight manner, when the holding body is in the expanded state.