portions 66 corresponds to the tapered portions 61 of the chassis 2a the chassis 2b has coupling grooves 8b 1 of a length l1 each of which extends in a direction perpendicular to the side wall 32b or 33b and in an inner direction of the chassis 2b and then extends downward also the coupling grooves 8b 1 extend from the front wall 31b to a middle portion in a direction of the rear wall 36b the chassis 2a has coupling grooves 8a 1 not shown having a symmetrical shape to the coupling grooves 8b 1 at two upper portions of the chassis 2a the coupling member has a length l1 and the basic structure is the same as that shown in fig4 in coupling and fixing the chassis 2a and 2b the chassis 2b is first placed on the chassis 2a on the front side of the chassis 2a then the chassis 2b is moved in a right and left directions such that the convex portions 56 1 are fit to the convex portions 59 1 subsequently the chassis 2b is moved or slid in a direction of the rear wall 36a of the chassis 2a so that protrusion portions of the convex portions 56 1 are.