1 to a first arm 6 the first arm 6 is mounted in a horizontal position rotatably by a vertical shaft 7 1 to a second arm 7 the second arm 7 is mounted in a horizontal position rotatably by vertical shaft 8 1 to a third arm 8 the third horizontal arm 8 is in a vertical position rotatably mounted to an arm support 9 by a horizontal shaft 9 1 and the support 9 is provided with a horizontal piece 9 2 bearing the third horizontal arm 8 horizontally the arm support 9 is fixed to a support member 10 protrudently provided on the backside of the seat 1 for the details of the drawing and retraction means in the unit box 5 and for the details of a cartridge system sheath in the box 5 reference can be made respectively to the disclosure of japanese utility model publication no 40626 1977 and to the disclosure of japanese patent publication laid open to public inspection no 96296 1978 and accordingly a detailed description of the means and the sheath is omitted since the unit box assembly 4 is of the construction described above it will be understood that the.