An insulator member includes a set of passages (2) (4) which, without intercrossing, extend through the main body (1a) of the insulator member. The set of passages (2) (4) includes an insulating passage (2) structurally arranged to be engaged by filiform elements (10) and an elongated anchor passage (4) structurally arranged to be engaged by the hook portion (21) of the stretcher device (20). Since the passages (2) (4) do not intercross, the separating portion (1d) prevents the stretched filiform elements from being subject to compression upon stretching of the stretching device.