A case body composed of upper and lower halves having a main wall in which there is defined at an intermediate portion thereof a pair of drive shaft insertion holes in right and left portions thereof, and the case body includes two pairs of upper and lower flange members inwardly projected from inner peripheral portions of the drive shaft insertion holes continuous with the main wall. A name plate made of a materials harder than the plastic material of the case body, is attached to an outer surface of the main wall including the right and left drive shaft insertion holes, and the name plate has one or more indexes formed on the outer surface thereof and has a pair of right and left shaft insertion openings defined therein. The positions of the shaft insertion openings correspond to the drive shaft insertion holes defined in the case body.