Selective connection of a first pair of output openings or a second pair of output openings opening in different directions with pipe fittings and closing of unused openings with closing plugs are facilitated by the use of a, substantially U-shaped removable stopper pin whose legs are inserted into the corresponding legs of fit-in grooves on both sides of the output openings and engaged with engaging grooves around the periphery of the pipe fittings and closing plugs.