element of the invention is a support plate 30 supported on the casing 21 by flat mutual support faces 31 and oriented in an axial direction the support plate 30 comprising on its edge a hook 32 facing the front and passing under the end hook 24 of the first flange 22 the support plate 30 extends on a full turn as previously the straightening sectors 20 bear towards the front a projection 33 for retaining the adjacent ferrule sectors 34 underneath another projection 35 can be noticed it is found under the second end hook 25 and its purpose is to support ferrule sectors 35 opposite the preceding slugs 37 are adjusted through the casing 21 in a radial direction and keep the support plate 30 supported against the casing 21 by engaging behind the latter to block it axially the hook 6 of the conventional casing 7 has been eliminated with the hook 32 of the support plate 30 replacing it such that a weak point of the casing 7 has been eliminated the hook 10 for the other flange 3 has not been eliminated completely since the second end hook 25 is placed behind a groove 19 of the casing 21 which ensures the same support functions for the flange 23 but it should be emphasised that the groove 19 more massive and less.