any suitable device an air cylinder operated by compressed air is shown in the preferred embodiment extending from a right hand end of the air cylinder actuator 17 is an output means in the form of a shaft 18 shown in an extended or open position the shaft 18 can be partially retracted into the air cylinder actuator 17 to a closed position not shown and extended to the open position shown in the fig1 in the direction of a double headed arrow 19 the shaft 18 is retracted under the influence of compressed air which is supplied to a power supply inlet connector 20 at the right hand end of the air cylinder actuator 17 coupled to the inlet connector 20 is one end of an actuator supply line 21 an opposite end of the supply end 21 is coupled to an outlet connector 22 of a control device 23 such as a three way valve the valve 23 has a vent 24 and a power supply inlet connector 25 the inlet connector 25 is coupled to one end of an inlet supply line 26 having an opposite end connected to a power supply inlet connector 27 the inlet connector 27 is mounted on an exterior surface of the left end plate 15 and extends through the left end plate to the inlet supply line 26 the actuator 17 the control device 23 and the associated supply lines 21 and 26 are enclosed by a cover 28 extending between the facing interior surfaces of the left end plate 15 and the right end plate 16 the cover 28 is in the form of a generally inverted v shape and has a pair of lower edges which are attached to opposed longer edges of the base plate 14.