Abstract: A mould comprising a first and a second half-part (15a, 15b) that are mutually side-by-side along a geometric centring axis (X) in order to define a moulding cavity (16). An internal wall (16a) of the moulding cavity (16) has a first and a second series of slots (17a, 17b) having the respective axially outer terminal portions (19a) respectively in the first and in the second half-part (15a, 15b). The slots (17a, 17b) respectively belonging to the first and second series are circumferentially offset from each other and have respective axially inner terminal portion (20a, 20b) in proximity to an intermediate reference plane (P). A junction line (22) between the first half-part (15a) and the second half-part (15b) has an undulating progression astride the intermediate reference plane (P), to form alternatively arranged crests (23), each intersecting the axially inner terminal portion (20a, 20b) of one of the slots (17a, 17b).