is approximately 3 32 inches thick and extends approximately 7 u215b inches from abutment wall 38 inside surface base 32 is approximately 1 u00bd inches wide at distal end 54 and continues at that width until it intersects a 3 inch arc with a center at inside corner of abutment wall 38 on front face 52 parallel to and approximately u00bc inch inboard from front face 52 first stiffening rib 45 runs from inside wall 50 to distal end of linear scale 28 first stiffening rib 45 is approximately u215b inch thick and depends from the top surface of top wall 14 to bottom surface of base 32 first stiffening rib 45 connects perpendicularly to second stiffening rib 46 that runs across distal end 54 of linear scale 28 approximately u00be inch in length stiffening rib 46 connects perpendicularly to third stiffening rib 47 that runs parallel to first stiffening rib 45 and whose outer edge is approximately u00be inch from front face 52 third stiffening rib 47 runs from second stiffening rib 46 back toward abutment wall 38 until it intersects a 3 inch radius centered at the intersection of front face 52 and the inside edge of abutment wall 38 and then angles back to inside wall 50 at a 45 degree outgoing angle an opening through base 32 has a proximal.