as used in this application , terms such as “ front ,” “ rear ,” “ side ,” “ top ,” “ bottom ,” “ above ,” “ below ,” “ upward ” and “ downward ” are intended to facilitate the description of the contact block of the invention , and are not intended to limit the structure of the invention to any particular position or orientation . referring to fig1 - 3 , 8 and 9 , a contact block according to the invention comprises a molded casing having a central core 4 with a front end 6 facing axially away from the core and a rear end 8 facing axially away from the core in the opposite direction . screws 9 hold the core components together . the core also has a first side 10 ; a second side 12 opposite the first side ; a third side 14 extending between the first and second sides ; and a fourth side 16 opposite the third side . an axially movable input shaft ( plunger ) 18 extends from a boss 20 at front end 6 . two holes 22 in front end 6 receive mounting screws s for securing the contact block to and against a panel ( not shown ), which typically has a hole through which boss 20 and shaft 18 would extend . rear end 8 also has two screw holes 24 for alternate mounting of a din rail clip 21 ( see fig1 - 13 ), which has a slot 23 adapted to embrace a din rail . alternatively , rear end 8 may be formed with an integral din rail slot . as seen in fig9 , an axially movable brush 28 within the core carries two brush contacts 26 at its opposite ends . the actuator for this brush contact assembly is conventional and comprises an axially movable contact basket 25 having an aperture through which brush 28 extends ; a balance contact spring 27 between contact basket 25 and the front face of brush 28 ; and an actuation spring 29 between contact basket 25 and the rear end 8 of the core . brush contacts 26 engage respective stationary contacts 30 carried at the inner ends of respective conductors 32 . a pushbutton operator ( not shown ) on the outer end of shaft 18 provides user input for actuation . fig9 shows an example of a normally closed ( nc ) actuator arrangement : inward movement of shaft 18 engages contact basket 25 and causes the contacts 26 , 30 to separate , breaking the circuit . fig1 a shows an example of a normally open ( no ) actuator arrangement : inward movement of shaft 18 engages contact basket 25 and causes contacts 26 , 30 to close , making the circuit . it should be understood that the invention is not limited to any particular type of internal actuator or contact arrangement , or to the type of external operator that provides user input to the actuator . referring to fig2 , 3 and 9 , each side 10 , 12 of the core has an integrally formed wiring compartment 34 defined by parallel tapered extensions 36 of sides 14 , 16 and an interconnecting lateral end wall 38 . the edges of extensions 36 and end wall 38 define a front opening 35 and a rear opening 33 for tool access to the wiring compartment . each end wall 38 has a pair of front wire guide ports 39 and a pair of rear wire guide ports 41 . referring to fig1 a and 9 , each conductor 32 extends laterally outward into its respective wiring compartment 34 , substantially dividing it into a front wiring chamber 40 adjacent guide ports 39 and a rear wiring chamber 42 adjacent guide ports 41 . thus , each front wire guide port 39 serves to guide the end of a wire into front wiring chamber 40 and each rear wire guide port 41 serves to guide the end of a wire into rear wiring chamber 42 . a terminal assembly for each wiring compartment 34 is connected to conductor 32 in one of the wiring chambers depending on the application : in rear wiring chamber 42 for panel mounting as shown in fig1 a and 9 ; in front wiring chamber 40 for din rail mounting as shown in fig1 . each terminal assembly is conventional and comprises a terminal screw 44 that engages a threaded hole 46 in conductor 32 and preferably includes a lock washer 49 beneath the head of the screw and a roughened wire clamping washer 48 , which can clamp one or two wires inserted through the wire guide port ( s ) 39 or 41 . referring to fig2 - 7 and 14 , each wiring compartment 34 is provided with a protective molded cover 50 that snaps onto casing 4 and is held in place by four corner snap tabs 52 that respectively engage four recesses 54 in casing extensions 36 . the sloped edges 56 of extensions 36 act as ramps that spread opposing tabs 52 apart as cover 50 is pressed onto the wiring compartment until the tabs reach and engage recesses 54 . cover 50 can be pried off if necessary by inserting and twisting a flat screwdriver blade or similar tool in the gap 58 between the casing and the cover . cover 50 also has a solid end wall 60 , a u - shaped terminal access opening 62 in the opposite end wall 64 and a side - by - side pair of wiring ports 66 in its lateral end wall 68 that are axially offset toward opening 62 . cover 50 has a universal configuration that enables it to be installed over either wiring compartment 34 in either of two orientations as required by the mounting application , i . e ., as a function of which wiring chamber 40 , 42 houses the terminal assembly . for panel mounting , cover 50 would be installed with its opening 62 facing rearward to enable access to terminal screw 44 in rear wiring chamber 42 ( see fig1 , 1 a , 2 , 3 , 8 and 9 ). for din rail mounting , cover 50 would be installed with its opening 62 facing forward to enable access to terminal screw 44 in front wiring chamber 40 ( see fig1 - 13 ). in either orientation , cover wiring ports 66 are aligned with one pair of wiring chamber guide ports 39 or 40 so that one or two wires w can be inserted through the aligned ports and into the appropriate wiring chamber , where they can be clamped by tightening the terminal screw 44 ( see fig1 a ). the positional versatility of cover 50 is due to the biaxial symmetry of the mating portions of the cover and the casing , as is evident in fig2 - 9 , 13 and 14 . the mating portions of the casing , which are identical on each side 10 , 12 , comprise ( 1 ) the laterally protruding contoured edge 37 of each side extension 36 , which has a double reverse curved and straight profile , and ( 2 ) the four tab - receiving recesses 54 . these casing features collectively are symmetrical about a longitudinal axial plane , i . e ., one containing the axis of shaft 18 and bisecting both wiring compartments 34 . they are also symmetrical about a plane perpendicular to that axial plane and bisecting both wiring compartments between guide ports 39 and 41 . the mating portions of the cover comprise ( 1 ) its two contoured edges 67 , which are substantially congruent with the contoured edges 37 of the casing , and ( 2 ) its four snap tabs 52 , which are axially and laterally spaced to fit into recesses 54 . these mating cover features collectively are symmetrical about the same two perpendicular planes . while a preferred embodiment has been chosen to illustrate the contact block of the invention , it will be understood by those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined by the appended claims .