of the other center core 42 at the upper side of the spacer 43 and stops a rotation relative to the center core 42 at the upper side if there is another spacer 43 at a still upper side than the center core 42 at the upper side the top end of the pin 43 b enters in the large diameter hole portion 43 d in the spacer 43 at the upper side meanwhile because at an upper side the uppermost center core 42 has no center core 42 to stop a rotation thereof at the upper side is arranged a thin top spacer 43 u2032 in thickness where the pin 43 b is protruded only downward as described above the pins 43 b of the spacer 43 43 u2032 are fitted into the small holes 42 e of the center core 42 and the spacer 43 43 u2032 is fixed to the center core 42 thereby engaging the spacer 43 43 u2032 with the center core 42 therefore a plurality of center cores 42 are assembled integrally fig5 so that the rotational stability of the center core 42 can be improved further in this instance the distal end of the pin 43 b of the spacer 43 43 u2032 protrudes from the center core 42 as illustrated in fig5 considering the pins 43 b protruding upwardly from the spacer 43 as shown in fig7 the protrusion amount b of each pin 43 b that protrudes from the corresponding small hole 42 e becomes smaller than the depth a of the recess of each large diameter hole portion 43 d this can also be said to the pins 43 b.