a relative to the roller 20 a and is held away from the flange 22 a by a packing piece or spacer 24 a inserted between the flange 23 a and the lateral face 21 f of the core 21 opposite to the face 21 d thus because the flange 22 a is partially engaged in the depression 21 c the flanges 22 a and 23 a that are flat define between them a volume with a thickness e for receiving the roller 20 a offset relative to the mid plane p 21 of the core 21 the roller 20 a is slightly thinner than the value of e in the same manner the roller 20 b is housed between two flanges 22 b and 23 b of a pair of flanges mounted either side of the core 21 the flange 22 b being partially engaged in a depression 21 g made in the thickness of the core 21 on the side of the face 21 f a packing piece spacer 24 b makes it possible to hold the flange 23 b at a distance from the face 21 d and from the flange 22 b which makes it possible to define a housing to receive the roller 20 b offset relative to the plane p 21 opposite.