One surface (32) of the transmission rod (29) is reduced in profile to its core cross-section (35). Due to this, connection to the adjusting members is possible in only one relative position. The transmission rod has a square cross-section, with one corner removed. In this point, the circumference follows a core circle, with the square sides tangential to it. The cross-section of the aperture of the adjusting member is formed to fit the cross-section of the transmission rod.
